Welcome, Kat. You'll find that there are a number of folks here who are more than willing to help.
When you post a request for assistance, it is important that you include the exact Mac model you have, and the exact Mac OS you are using. Also, let us know if you have ever done any disk cleanup/maintenance/repairs from a software perspective, ie, you used software to help with those tasks (and also if you did any of that on your own). And so, so important: hopefully you are making backups to an external device.
Once again, welcome!