Hi @mizkendall3939 :)
I’m a girl, so this will be general fitness tips, but I hope some is valuable to you anyway.
Stage zero is as @Quagmire says, get a physical just in case.
First off, when you ramp up your activity you need to also keep an eye on what you eat and eat good stuff. For being in shape you will want to add more protein into your diet, chicken breasts are good, eat plenty of fruits and veggies, and try to not eat too much carbs cause they really are everywhere (chips, breaded items, sugars, etc) -> but don’t let this stop you from having some from time to time!
There are a lot of ways to get in shape depending on what you like. Do you have an interest in martial arts? Many places offer a trial class so you can go and see if you like it enough to sign up. If you are the type of person who likes having a commitment in order to guarantee you’ll work out, courses are great, because you paid for it, you don’t want to bail out!
There are plenty of things you can do for free on your own time though. Even something as simple as taking a run is good for you. It will help increase your stamina which helps in every other athletic activity. Going to a gym is a good idea because education is an important part in fitness, you don’t want to injure yourself by doing an exercise wrong or using a machine incorrectly, and the trainers can offer you plenty of tips and advice.
You can’t really develop only a certain set of muscles though. You have to be healthy all around to have really defined muscles. Remember, fitness is 50% effort and 50% genetics, some people simply can’t put on weight, some can’t lose it, some can’t develop tight ab muscles. But you can do stuff to increase your muscle gain. Exercises that focus on muscle work such as weight lifting and kettlebells are good for developing muscles more than cardio like running, but a balance of both keeps things interesting and doesn’t tire out your body. Alternating one day of strength training and one day of cardio is a nice balance.
Results equals effort. I’ll say it again: results equals effort. If you work hard, you will become fit. If you slack off or choose to eat a bag of chips or an extra slice of pizza (and regret it), you will become fit at a slower pace. I’m not telling you to only eat salad for your life (that’s unhealthy anyways), but it’s important to understand how eating affects weight gain. It’s important to eat more protein because that’s what your muscles need to build themselves back up again (working out is really causing small micro-tears in your muscle tissue which then gets repaired, hence you feel tired and sore). It’s important to know that a deep fried chicken piece like KFC is not the same as a chicken breast from one of those prebaked chickens at the store (or even better, a poached breast).
Because of your age, I’m not sure if a gym would let you in as a member, or if you have the money to pay for such activities. You could ask your parents to sign you up in a martial art class, or put some savings towards it if you have some. But even if you have zero money there is a lot you can do to become healthier. Start by going for a jog or a run, that’ll always help.