You need to tell us more information:
1. What exact Mac model are you using?
2. What exact Mac OS are you now using?
3. How much memory do you have inside the machine?
4. What is the size of the internal hard disk? And how much free space is left on the drive?
5. Have you ever done any disk cleanup/maintenance/repairs?
6. Do you have either MacKeeper or CleanMyMac installed? If you do, get rid of them IMMEDIATELY, as they do more harm than good.
7. Do you have any anti-virus software on your machine?