iIs cool if you are poor, I was/am poor too but here are some more tips lol:
-I don't know where you live but H&M is a world wide store that mainly sells trendy clothes for very cheap, so maybe try looking in there or shops similar to it. You could buy online but sizing is always a risk when buying online.
-Invest more in jackets and sweaters since you get the most versatility with them. So don't be afraid to spend a lot on nike or north face jackets case they are good brands and look really nice. Try to stick to lighter windbreaker type jackets cause they can be worn during fall, winter, and even spring without looking too weird. They also last the longest cause they require less washing.
-Don't buy to many branded shirts cause those are expensive and you can't wear to often since people will notice. Stick to buying solid or light pattern shirts for versatility. These are also usually cheaper. And if you haven't started gym celling yet and are afraid of your skinny arms ill recommend long sleeves. They hide your arms and can be worn year long granted they are thin enough.
-For pants stick to these three colors to get the most use of them. Black, Dark blue, and Light Blue. These match with anything and are super versatile. Invest a lot in these too although not yet cause you are still growing, ill recommend Levis although those do get pricey. Also some nice black chino pants and darker khaki chino pants are good too.
-For Shoes just stick to what is in so like Vans, Nikes, and Addidas. Most of there basic popular models are between 50-60 dollars and should last a few months. Vans are the cheapest of the three and you could find some decent ones for like 40 easily. Try to stick to black/white shoes cause those are the most versatile.