I'm currently using the OnGuard Bulldog DT U-Lock. I hook it through the rear wheel into the cargo rack, which prevents the rear wheel from turning, then secure the cable to something solid nearby and attach it to the U-lock. If I'm parking somewhere that doesn't have an anchor point, then I loop the cable through the front wheel and attach it to the U-lock, which I (again) hook through the back wheel and cargo rack. It's good to keep in mind that any lock system can be defeated if someone is determined enough, but this has worked well for me thus far and provides me with peace of mind. Sidenote: I picked up this lock combo set for $24.99 in late June from Amazon, so the price has definitely gone up, but I'm sure you could probably find it cheaper elsewhere.
Alright so here's the dealio. The police in Davis do pretty much nothing for bike thefts, which I still don't understand, but whatever. Definitely don't buy a super expensive $600+ bike, but still don't buy a cheapo Walmart bike. Those things are legit death traps. I suggest going to the Bike Auction every Fall and Spring quarter. There's the Davis Bike Collective, too. They sell used bikes and parts. Oh, there's also places like the Green Bike Depot. Basically, find a dad bike.
Even if you decide to get a nice, new bike, just lock it up well and don't be careless about where you park it. It's better to park it next to a large group of bikes than park it by itself somewhere. As for locks, get a U-lock and a cable. I use something like this: https://www.amazon.com/OnGuard-Bulldog-U-Lock-4-Foot-9-06-Inch/dp/B005YPKC5O/ref=sr_1_10?dchild=1&keywords=Onguard+Bike+Lock&qid=1625774939&sr=8-10
This is a pretty good video about how to lock your bike: https://www.youtube.com/watch?v=DAZQwM-2O7M
I've commented on a lot of your posts so I'll just give you a quick guide here (or maybe I should make a post idk):
Theft Prevention
Here is how you should lock your bike: https://www.youtube.com/watch?v=P77drF8BEcg
U-lock through the frame and onto the metal racks!
Some locks come with a loop cable. You use this to secure the front wheel to the u-lock. https://www.sfbike.org/wp-content/uploads/2014/02/Locking-Techniques-Rear-Triangle-Thumb.jpg The back triangle is kind of hard to use at UCSB. Anywhere on the frame will do.
Cable locks suck and can be cut by $10 clippers from Home Depot. Cheap U-locks can be opened by a ball-point pen. Buy an OnGuard or Kryptonite lock.
Here are two options for locks:
http://www.amazon.com/OnGuard-Bulldog-U-Lock-4-Inch-9-06-Inch/dp/B005YPKC5O/
Be warned, longer u-locks (9 inches) can be broken through leverage. This may damage the frame so thieves seldom use this option.
Now that we've gotten securing your bike out of the way.....
Where should I buy my bike from?
Your local bike shop/REI will be overpriced. Check when their clearence sales are, because the deals are usually very good.
Craigslist is the preferred option for buying a used bike. I dislike dealing with BS so I didn't use this option, but I did shop around.
Online? Not really sure. There are plenty of botiques like PureFix (no..... their bikes are insanely heavy and crap). I have used Bikesdirect.com twice and have been satisfied both times. The website sucks and it'll take some browsing around, but you should find tons of options.
What kind of bike should I get?
A road bike. You don't need gears either at UCSB (flatzone), but it doesn't hurt to have them. A fixie is hard to ride if you are inexperienced, but a single-speed (with breaks) is the easiest thing in the world. A fixie with a FLIPPABLE HUB (you flip the backwheel, youtube) can be used as both a single-speed and a fixie).
The difference between a fixie and a single-speed is that when you stop peddaling on a single-speed the pedals stop with your feet and you can't peddle backwards. On a fixie the pedals will keep going and you can pedal backwards. Also, most fixie's come without breaks as a skilled rider can stop the bike by pushing back on the pedals.
The material of the bike... Carbon-fibre is too expensive and generally not useful at UCSB. You are better off with aluminum or chromoly (steel is heavy, but at a lower budget you may have no choice). Modern aluminum bikes are very sturdy and durable, also chromoly is a classic material for roadbikes.
At a low budget here are some recomendations for a bike:
http://www.bikesdirect.com/products/windsor/hour-ltd-track-fixie-bikes.htm
http://www.bikesdirect.com/products/dawes/fixie-track-bikes-streetfighter.htm (friend has this).
http://www.bikesdirect.com/products/dawes/lt_sport_al_xiii.htm
Literally countless options between $200-$300. At $400
As for bike sizing. Use this website. http://www.ebicycles.com/bicycle-tools/frame-sizer
Here are some finer details on how to ride a road bike (to protect your knees!):
https://www.youtube.com/watch?v=8sMwG6PLh5o (ignore the clips stuff pay attention to posture and how he mounts and dismounts the bike when stopping, you aren't racing). Make sure you give your knee room to stretch.
That's about all I can think of. Good luck.