Hey Hafeezusman, what country do you reside in? If you're in the united states, then many US citizens flock over to coinbase. However, if you want to get your BTC for lower rates and without the high fees that coinbase impedes on you, then you should consider signing up on Gemini.com instead.
Another great resource would be Kraken
If you need further assistance, then feel free to ask me anything!