Don't expect that your first game would be a great success. It's a learning experience.

Needless to say, having a small scope is very important. Aim to have 15-30 minutes of gameplay maximum. I can't stress this enough. Better to have a short but sweet game, than a long but "buggy" game which has a lot of content padding.

I put "buggy" in quotation marks because players can say features are bugs in your game. :)