My 21st birthday

First well done for giving yourself plenty of time! It's not a quick journey and it can be frustrating, but equally enlightening and rewarding.

Simplest way forward is to add your details and goals in to MFP and it will suggest what you eat to achieve it. It can be a little cranky at times I find but basically you want to eat 75% of your TDEE. Don't be tempted to go lower please. That can have negative effects and hard to maintain.

What you want to do is create new habits. So you are looking at what you eat, drink, when you sleep and what exercise you get. Then looking to see where you can make small changes in the right way. For example, dropping sugary drinks, drink more water (nice with lemon squeezed in it).

Exercise, hitting the gym is a great start. 4 days a week I would likely have a 3 day weights program and a day for cardio (HIIT) and maybe higher rep lifting. I appreciate its a lot to take in, you have a lot of options and it's important to find stuff you enjoy (easier to stick to!).

Maybe spend a week in the gym and see what you think. Me or many people here can point you in the right direction for lifting programmes and so on. Good luck!