A lot of helpful advice and really what I have to say is just echoing their counsel.
1) Put Jesus Christ first. If you do that all things will be added unto you. This means praying every day at least once but preferably twice. Ask Jesus to show you who he is and that you mature in Christ.
2) Yes be very selective about your friends. Notice I said "friends" and not acquaintances. Feel free to have a lot of acquaintances and be friendly to everyone (except false brothers and sisters, ignore them) but have very few people you would consider friends. A good friend is extremely hard to find. The litmus test is would they sacrifice something of themselves (time, money, etc.) to help you without any question or hesitation. You actually can notice this trait in the little things they do if you pay attention. Also notice how they treat other people when they're around you and how they talk about other people.
3) If you have a lot of debt be very careful about adding more to it in graduate school. These days you can put up maybe half a million in debt with college and graduate school and that's not counting interest. Be very wise here.
4) Don't try and keep up with the joneses. If someone buys something or has something you can't afford don't try and keep up with them. Stay in your lane.
5) Instead of trying to find the "perfect" person work on yourself and your own perfection/sanctification. No one is perfect and the more you develop and mature the easier your life will be with all people, but perhaps most importantly with any future spouse.
6) Take risks. If you're risk-averse like I am start with little ones. Don't be afraid to make mistakes.
7) You're young. It's hard to appreciate where you are when you are where you are but this is an opportune time to learn about yourself - your likes, loves and dislikes. Go to museums, parks, wherever. Explore yourself.
8) Develop a hobby if you don't have one. It build self esteem to acquire mastery of a skill, no matter what it is. It's something you can hold on to down the road when things get rocky.
9) Finally, life isn't easy but it is beautiful.