Like Dark Pulse said, make a bad map, you can always return to it and fix what you don't like about it. There's no rule saying you can't go back to it. I made bad maps, it's how we figure out how to make the good ones. ;)
Something to keep in mind is to keep things small. You don't have to make a megawad. It's far easier to make single maps and smaller map sets. Save the megawad thing for a later time. I think I made the mistake of making a 32 level megawad from the start. Keep it smaller and work on what you find is enjoyable.