Ok so you need a wallet no an account
A wallet is something you use to recive and send btc
To buy btc you just need to go to a market online and buy there
No there is no intrest rate you profit when btc prices go up
Withdraw? You can sell your btc on the market again to recieve your cash
You can search online what market you want to use there are also apps but im not sure what country you are in so just look it up