You can buy and trade Bitcoin in the exchanges.
Example: "Binance"
First, you have to make an account in the exchange or e-wallet that is available to your Country.
Example: Binance
2nd, Go to "P2P" section then click "buy" (you can choose a specific currency or choose BTC) then you can see the list of payment method.
Just follow the simple instructions provided in your Binance app and you will immediately get BTC.
If you want to hold your Bitcoin for a long time investment, you have to transfer it to a safer wallet.
I recommend to use Electrum because that's what I'm using, just make sure to keep your private key and phrase where only you knows it. If you have a hardware wallet, it's much safer.
Then if the time has come that price of Bitcoin is so expensive, you have to transfer it again to exchange in order to trade it.
You can see the list of top exchanges here: https://coinmarketcap.com/rankings/exchanges/