I am storing in an app called BTC wallet. I noticed that every time I did a transaction, my address would change. Is that normal?
You mean
https://wallet.btc.com/ If yes than they will create new bitcoin address whenever you want to receive bitcoin and it is normal. You will get all your bitcoins in your wallet which you can send out in future. But i will recommend you to use electrum desktop wallet as they will give you complete control over bitcoin address where you can receive bitcoin.
Web wallets would be somehow noob-friendly specially since hes just a starter and also he might store small amounts of bitcoin and web wallet or online wallet would do just fine i guess and he will surely discovered on using desktop or hardware wallets in the long run for sure.