Buying/selling bitcoin is very easy, but getting them at the right moment is really important. While buying bitcoin consider checking multiple sites and get bitcoin from the lowest trust websites, they are listed below.
There are many sources including websites from where you can buy and sell your bitcoin with your local currency.
For buying selling, you need to verify you identify through KYC, for some of them charge taxes on the basis of your country.
You can always exchange you bitcoin for any altcoins through exchanges. Join Binance for exchanging Cryptocurrency
coinbase include buying selling for BTC, ETH, Litecoin.
Local (In India)
Localbitcoin let you buy bitcoin with through banktransfer, paytm, cash on hand and many more. I personally buy bitcoins through this but others are also good.