Partners are always great in the “winter” because the stuff gets real if you run into problems. There usually is a trip advertised each weekend on 14ers.com that are good if you have the strength to keep up as they won’t babysit a fat noob, which obviously you aren’t.
If you are going in the summer, this is the best book for finding amazing hikes. Worth the money. Solo is okay if you have a good cell phone GPS app so you can record tracks and get back down if something goes wrong. https://www.amazon.com/Best-Summit-Hikes-Colorado-Little-Known/dp/0899974082
Critical summer gear list: Running shoes (boots are unneeded if you have strong ankles), Pufffy jacket, Rain jacket, Headlight, Cell phone with GPS app like peakbagger, Power bank, Lighter. Water bottle. Hat or helmet for class 3+ peaks Ice axe and microspikes if there is any chance to get on steep snow.
Everything are nice to haves instead of must haves.
Mountains are so personal, just like girlfriends. Find one that matches your dreams. Lots of gorgeous peaks out their of all shapes and sizes. HYOH.
A couple good resources to pick your “tinder” match: 14ers.com (free)
James’ book (cheap): https://www.amazon.com/Best-Summit-Hikes-Colorado-Little-Known/dp/0899974082
Rooting for you bro! Post pics after your trip.
The real class 3 section on Longs is a 15’ crux on top of the trough. Photo 12 on 14ers.com description. If you have a partner who can help spot you from below, the moves are quite doable even for a beginner. Be careful of afternoon thunderstorms so start early.
https://www.14ers.com/route.php?route=long1&peak=Longs+Peak
Once you get into 14ers, you get addicted quite quickly. Also consider James’ book that is well worth the $12. All of his peaks are gems and the writing is top quality.
https://www.amazon.com/Best-Summit-Hikes-Colorado-Little-Known/dp/0899974082
If you start looking out of Colorado, there is a huge variety of lists you can find on the free website or mobile app Peakbagger. For some people, it is particularly motivating to have that type of focus. Options such as state highpoints will get you excited to see the country.
I assume your English is nearly perfect since you are from the Netherlands. If you want to enjoy peaks in the United States, it is easy to fly to Denver, rent a car, and spend a couple weeks in the Colorado Rockies. July and August are great months with minimal snow on the peaks. No glaciers so you don’t need technical skills. Plenty of free places to sleep in the forests. The main thing you need for safety is a good GPS app like Gaia or Peakbagger so you can backtrack down to your car if the weather turns bad.
A few resources to see options in Colorado.
https://www.14ers.com/php14ers/14ers.php
https://www.amazon.com/Best-Summit-Hikes-Colorado-Little-Known/dp/0899974082
If you want bigger objectives, Chile and Argentina have 6000m peaks that can be done in December-February months. Examples include Marmolejo, Aconcagua, Ojos, and Mercedario. If you are in good shape you can do 6x6000m peaks in 3-4 weeks.