Need the following information:
1. What exact Mac model do you have?
2. What exact Mac OS are you using? And if it is a recent upgrade (like High Sierra), how exactly did you upgrade?
3. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You actually can do a good amount of disk cleanup on your own, and there are some excellent programs available (both free and commercial) that can assist you in those efforts. Along with that, how much free space is on your internal drive?
4. This is critical: are you making backups to an external device? If so, what software are you using for this?
5. What third party (ie, non-Apple) software are you using?
Once you answer those questions, we can try and help you.