how do i create a url for a wordpress site?

Do I need to get a web hosting package for the theme i m trying to sell?
If so which are the cheapest web hosting providers…if not how can I do it…

Yes you do need this.

There’s really budget hosting about but you will regret it (bad customer experience, downtime etc).

Best option is look at one of the big shared platforms like Blue Host - they are not expensive and reasonably reliable