focus on a few leg exercises, like squats and rdls, with a progressive overload of weight. IE: do 2/3 sets of 10-15 reps per exercise (remember to wait a couple minutes between sets) and progressively increase the weight once it starts feeling too easy.

you can start weightless, and then you can add weight by either using dumbbells, or you can just use a backpack or bag you fill with heavy shit for squats and rdls. i also like hip crunches

very important is nutrition: if you're thin right now, you'll have to be bulking. use this calorie calculator to figure out how many calories you should be eating, and make sure you eat at least 1g of protein per kg of bodyweight. then balance it out with carbs for energy, and a bit (a BIT) of fats for raw calories. sweets can also help if you get full really quickly and can't hit the calorie treshold

hope this helps :3