[Your title needs to have a summary of what you are asking rather than just 'help' - but I suspect this is your first post so maybe the mods will probably let you off gently and modify it - Message to mods - could you screen subjects for single words and pop up a warning to the poster if any of the regulars e,g, 'help' occur on peoples early posts?]
Anyway to help you, some questions:-
1. What OS are you running?
2. How much RAM have got you installed?
3. How full is your hard drive?
4. Has this started happening after you installed something?
5. Have you done any of the maintenance things, like zapped PRAM, run Onyx or Mac Janitor etc?
6. Have you recently added or removed RAM?
Sorry for the questions, we need more info.