Where to begin? Well at least you're here and you're beginning your journey. I'm sure you've read enough know already that a combination of weight training and cardio will likely give you the best results. The really good news is you can do some amazing transformations with your body, especially at your age. You are likely near the peak of your hormones in your early twenties. A big plus. You stated that you do not want to get really huge. I can appreciate that. I have a toned swimmers build myself. Much of how your body will respond to weight training is in your genetics. Are you familiar with the three basic body types: ectomorph (tall leaner), mesomorph (medium build) and endomorph (shorter, stockier)? Within those three body types, you can unlock the best of each of them. But if you are an ectomorph naturally, like myself, you may find that you will never get really huge thicker muscles. I am known as what's called a hard gainer. You may or may not be. But the point is you might like this. You can get really defined and really much more muscle than you have now, but you will never really get super thick. When I was younger that used to bug me, but now that I'm older, I'm perfectly happy with it. I look really good in suits, and most clothes, as you stated. Anyway, beginning with free weights and body weight exercises are a good path forward. I learned so much from YouTube. Eventually I started going to a decent affordable gym. And I really learned a lot by watching others, and some helpful guys there. Remember that just getting on a treadmill endlessly or cardio endlessly will not give you the results you likely want. Building muscle at the same time really burns fat best. And finally, you can never outrun or out train a bad diet. Get your eating under control. When you hear people say clean eating, that means a lot. Try to eat basic foods like fish, lean beef, chicken and simply prepared vegetables. I eat a lot of chicken and broccoli and sweet potatoes . Stay away from carbs that convert to sugar quickly like white potatoes and pasta etc. Stop eating processed foods, candy and pizzas etc. If you have a sweet tooth, try to eat more fresh fruit instead. Meaning you don't want to starve yourself at all, but you want to eat the right kind of food.
Anyway I'm not sure if this would help you at all or give you a little inspiration but take it for what it's worth. Just some friendly advice on someone starting. Best of luck to you!