It depends on where you come from. Some places are restricted or have no support for buying and selling. So it's very hard to suggest to you which service of app to use exactly but here are some options

1.  Buy and sell directly to trusted forum members in this section  - https://bitcointalk.org/index.php?board=53.0
Use escrow if someone is not an old trusted members in the forum.

2.  Use P2p exchanges, like the ones in this list - https://bitcointalk.org/index.php?topic=5224711.0

3. Use centralized exchanges that support fiat like Binance, coinbase, Kucoin. All have apps and not all fiat money is supported in such platforms. Make sure you read terms use.