The best trading app that I would prefer to you is COINBASE. Coinbase is the largest Bitcoin exchange around the world. It is a digital currency broker exchange that you can buy and trade bitcoin and also with other currencies, it is also allows you to buy crypto using fiat currencies. In addition ,it is one of the safest wallets that we can use and it has an offline cold storage so that it cannot be accessed for all the hackers when in the cold storage. Coinbase is a user-friendly so that it wouldn't be hard for you as a first time trader and you can buy currencies using credit card or debit card.