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I know that feeling! Okay, here is what I've learned to do over the years to cut corners and save dollars. It's gonna be long, so take what you want and leave the rest. :)
The first thing I'll suggest is to start learning to budget now. I spent two decades living paycheck to paycheck with WAY too much debt - still almost am - but am working my way out slowly. Part of the reason I lived paycheck to paycheck is that I never knew 'where I was' with money. I would end up overdrafting because I forgot about a check I'd written, or about an automated payment coming out. You can do this with a spreadsheet, but if you find that difficult, YNAB is a very popular program and for good reason - it's simple and straightforward - if you're willing to put in a little time learning the basics.
YNAB offers free live classes on all sorts of subjects, and your best bet is to use a weekend or two to sit down and go through the classes. In fact, even if you don't end up using YNAB, I'd suggest taking those classes, because a lot of what they teach is just as applicable to budgeting with a spreadsheet. Don't worry, they're not dry and boring - the teachers are very good and most of them have had experience with living without a budget, so they really make it relatable. They also take time at the end of each class to answer questions. You can take the classes as often as you like, and at the end of each class you're emailed a link to where you can watch a non-live class in case you want to go over it again with the ability to pause, rewind, etc. As well, /r/ynab is a very friendly community full of people who are all happy to help.
The cost of the software is a little pricey - $60US. Unfortunately they just recently announced they won't be participating in Steam sales anymore (boo!) but they do have a referral system in place that can make it a tiny bit ($6) less expensive, and the ynab community keeps a promo chain thread stickied (current post here) where you can use someone else's promo code to save $6, and then post your own so that someone else looking to save $6 can use yours and you get another $6 back. There is a small chance of getting it for free if you take those classes above - at the end of every class they give away a copy of the software. And you can retake the classes as often as you like.
Second, if you can, devote some time to getting the best deals. Some grocery stores will price match - find out which ones will, and what they need for proof. I found that Staples isn't worth trying, for instance, because they'll argue whether a product is 'the same' or not, but at Wal-mart I have brought in a page I printed out of a flyer which showed the date and they accepted it, and they've also accepted screenshots I've taken on my phone. I've seen people using stuff like iPads and it looks a lot easier, but I certainly can't justify the expense for just that! But if you have one, that's another way to do it. For going through flyers - you can get them in the mail, you can subscribe to them through email, you can use something like RedFlagDeals' flyers page, or use an app on your phone. Personally, I like using apps because it keeps it all in one place. I use android, but here's some of what I use to help me shop better:
YPShopwise for deals and flyers - you can view deals in categories but they also have a flyers section, and you can mark certain flyers as 'favorites' so they show up on the 'front page' of the app.
Stocard - an app to scan in all the different loyalty and points cards you have for different stores/businesses, and keep them in one place. Before I had this I used to inevitably forget to bring my points cards with me, now I always earn points. Sometimes the points add up so slowly that it almost feels not worth it to collect them - but you might as well if you're there, right? And other times, they can help you do something more often. I love my Scene card for Cineplex because movies are one of my few entertainment expenses, and it's pretty easy for me to get enough points to get into a movie for free - which is awesome when I'm having a really tight month and otherwise wouldn't be able to treat myself to a night out!
Price Cruncher - you know when you're standing in a store staring at three different version of a product, all different prices, all with different weights and/or volumes and/or counts? This type of program lets you type in the details of two different products and find out which one is cheaper. So if you're looking at say 481 square feet of something for $18.99, OR 384 square feet of the same thing for $11.99, you can punch it in and find out which is cheaper. I have used programs like this a lot over the years, and found it a huge help, as the deal that you think would be the cheapest isn't always the cheapest, and I can't do math in my head for the life of me. The other thing this is nice for is figuring out what's worth buying in bulk. You can save a lot by buying stuff that's not pre-packaged, but it's not always true.
If you are okay with second hand stores, you can save a lot in many situations. When setting up my household, I stocked up on a lot of kitchenware at my local sally ann. By picking through, I found a lot of stuff that looked near new, and yet it was still cheap, like 10c/pc for silverware and stuff like spatulas, etc. I still make occasional trips in order to save myself money - for instance I buy my shoes second hand from Value Village. Yes yes I know, some people will freak out at how 'gross' that is BUT by going looking before I need new shoes, I have been able to find shoes that are pretty much - if not actually - brand new, for around $15. I buy them before I need them, put them away in my closet and when it's time for a new pair, I can discard the old pair and pull out the new - by buying ahead, I don't have to run out and buy the first thing I see. I do the same with purses, and again, by looking ahead and taking my time, I can easily find a purse that is near-new or brand-new for about $7.
For cooking - well, unfortunately I'm a pretty big meat-eater so I miss out on a lot of savings that way. But fortunately there are a ton of subreddits you can explore, if you haven't already, both in the food category and other categories - do some exploring, find out which ones can help you stretch a dollar. For example you could start with /r/frugal, which has a list in their wiki of many other savings related subreddits, and check the sidebars of those subreddits for more subreddits.
The more you can cook at home, the more you can save, and of course the better food you can cook at home, the easier it is to talk yourself into doing so! I suggest looking at /r/slowcooking and /r/PressureCooking - two ways to save yourself time and money while still cooking a delicious meal, often in just one pot. Having tried both, I am partial to pressure cooking as it's a super fast way to make delicious meals, but there are pros and cons to both sides, so check them out and see if either appeals to you.
And lastly, a warning - be very very careful if/when you start getting into credit cards and such! I was stupid and I'm still paying for it, and I'm in my late 30s. The people that tell you that you only put on your credit card what you can definitely pay off the same month are absolutely right, because the more you put on, the longer you take to pay it off, the more you pay on top of the cost of the item. And if you're using credit cards to get by, then let's face it, you're probably not going to be able to pay more than the minimum payment each month. Rack up just $1k on a credit card at 18.8% (which is a middle of the road interest rate) and if you pay $25 a month you're going to spend five years paying off that money - if you don't use that credit card again for more stuff that you put on for more than a month! It's a vicious cycle, and if you can avoid it to begin with, you'll save yourself a lot of money and stress. Whereas if you can use it responsibly, a credit card can be another way to save money - there are cards that will give you cash back for using them. Pay off your balance every month and they are paying you to use their credit card!
Good luck - you're holding down a job at 18 and living on your own so you're already a mile ahead of where I was at your age. :)