Need to know the following information before we can help you:
1. What exact Mac model do you have?
2. What exact Mac OS are you running?
3. Is it Apple'sMail program, or another EMail client?
4. Have you ever done any disk cleanup, maintenance. and/or repairs from a software perspective? You can actually do a fair amount of disk cleanup yourself (including the permanent removal of deleted EMails), and there are also some excellent products available that can help you with those tasks.
5. Are you making backups to an external device? If you are, what software do you use for that?