Read the General problems FAQ before asking questions. If you still need help, post your specs (speccy screenshot), OS, emulator version number and details of what's wrong.
Please contribute to the wiki if you discover any inaccuracies or have relevant information to append.
READ THE WIKI BEFORE ASKING QUESTIONS LIKE:>Where do I get games
OR>What is the best emulator for...
Check out the wiki for the emulator you're using if you run into trouble, there may be a solution there too, often including recommendations for optimal game settings.