If you are just starting out as a painter I would do some research on methods of canvas priming and try them all out at least once just to see what it's like. Sometimes you will need to use different methods for different projects. Sometimes you will find that one method is a lot cheaper than another, or will produce better work, or work that will last longer.
If you are hoping to sell your work, you might want to think about how you will ship it to buyers as well. If you paint on unstretched canvas that can make shipping much easier, but it can also make it more difficult for the buyer on the other end.
But, if you are just starting out you can quickly realize that properly shipping an already stretched canvas might double the price of your work, which isn't going to encourage many sales.