A couple of questions first, before I recommend some excellent free programs, and an excellent commercial one:
1. What exact Mac model do you have?
2. Have you ever done any disk cleanup/maintenance/repairs using software?
3. This is
critical: are you making backups to an external device/ if so, what software do you use for that task? Even if you use the software I recommend below, it is imperative that you have a backup.
4. What software is telling you all those statements you mentioned?
OK, first, you can actually do a lot of disk cleanup on your own. One primary place to look is inside your downloads folder. Another resource "hog" is deleted EMails. When you delete an EMail, it is not permanently removed from your machine. Depending on whcih EMail program you are using, there are ways to have deleted EMails permanently removed. I use Outlook 2016 as my EMail client, and I am constantly have deleted EMails permanently removed. It is easy for me to do it, but I don't know about other EMail programs.
Here are some excellent freeware programs you can use to help you out:
Onyx - Available from here:
https://www.titanium-software.fr/en/onyx.html
Rock Solid, has been around a long, long time, and has had many positive reviews. I use it religiously (along with a commercial program entitled Tech Tool Pro). I would not be without either of them. In fact, I will not upgrade to a new Mac OS until there are compatible versions of both of them for the new OS (along with 4 other critical programs that I have). Note that there are different versions of Onyx available, for each different Mac OS. You would need to get the correct version for the Mac OS you are using..
AppCleaner - Available from here:
https://freemacsoft.net/appcleaner/
Another stellar performer! As you can see via that link, it will get rid of most stuff associated with an application that you wish to delete.
GrandPerspective - Available from here:
http://grandperspectiv.sourceforge.net/
Nifty graphical way to see which files are on your drive. Good way to identify large ones that you might not need.
Malwarebytes - Available from here:
https://www.malwarebytes.com/mac/
Good to use if you see adware and/or suspect you might have a virus. It also is good to run it every so often, even if nothing is wrong.
ClamXAv - Available from here:
https://www.clamxav.com/
More extensive than Malwarebytes in terms of finding viruses. You can use it in demo mode, and it still has just about full functionality.
Then of course you can use Disk Utility to check out your drive and/or do repairs. It's not as extensive as Tech Tool Pro (
https://www.micromat.com/products/techtool-pro), Disk Warrior (
https://www.alsoft.com/diskwarrior/?gclid=EAIaIQobChMI9qzeqcSF2gIViWp-Ch0NngwsEAAYASAAEgJg7_D_BwE), or Drive Genius (
https://www.prosofteng.com/drive-genius-mac-protection-software/), but it gets the job "mostly" done. I use Tech Tool Pro, and in fact it is the only third party one so far that is fully compatible with High Sierra (that's the OS I use on both of my Macs)
However, 2 caveats with using Disk Utility:
1. A backup is
mandatory.
2. You need to run it in "isolated" mode. For that, you would boot your Mac into the (Hidden) Recovery HD partition. This link explains how to do that, and what is on that partition:
https://support.apple.com/en-gb/ht201314