What advice would you give to a 21 year old specialist, to make the most out of life and the military?

Sounds like you need some life skills.

Find yourself a mentor. Someone who you want to be like when you grow up. You don't need career advice, you need mentorship from a decent human being and that will help you become what you will become. Don't expect that you'll ever know what you want to do in life until your 40s or 50s, but a good mentor can help point you in the direction of your talents. Once you try out enough new things you'll discover what you like and uncover what's important to you. You're never going to have have some moment where you have an epiphany and discover the perfect job that will make you happy. It's a puzzle that you slowly piece together.

CPL (Join to see) Your profile has you in the Reserves. What is your civilian job and do you like it? Continue your education by taking required courses until you can decide exactly what you want to do for a career. Do your research, talk to career counselors, etc. Decide on a course of action, plan, make goals, and carry out your plan. Plans are subject to change, so make course corrections as needed. You will make mistakes along the way, but you will learn from them. Good luck.

Mission first. Learn your mission and the knowledge and skills needed to do the mission.
Integrity always. Be honest with your subordinates, superiors, and yourself. Tell the truth when it’s easy and natural, but also tell it when it’s difficult and possibly against your self interest.
Education is important, but gaining the right knowledge and skills is important. Take full advantage of training available to you through the Army. Do all of the leadership training you can. The Army is good at building leaders. You’re on the first step now. Learn all you can and keep working at it.
A 4-year degree isn’t necessary for success in life. Technical or trades education can be the pathway to good paying jobs. This includes IT certification.
Find a mentor to emulate and advise you. At your rank, it may be a SSG or SFC who’s approachable and well respected.

Complete your RP profile and ask more specific questions. You’ll get better advice.