Whatever you do, DO NOT use MacKeeper! That software will cause much more harm than good. If you (god forbid!) have it installed, get rid of it IMMEDIATELY!
There are a number of safe, effective ways of cleaning "junk" off your Mac. Part of that effort requires work to be done by you, and a continued effort on your part. The first thing is to get rid of stuff you absolutely no longer need. One source of this are updates to software. Whenever I obtain a new update, I remove not the previous update, but the one prior to that. Also, if you have any documents (Word docs, Excel spreadsheets, etc. (or the equivalent ones with other suites/programs)), get rid of them. Then, of course, get rid of any duplicate files/folders you have.
One excellent free product you can use is Onyx. This venerable gem has some useful disk cleanup functions that will help. However, you need to obtain the correct version, as Onyx is OS-dependent.
Another potential source of clutter are EMails. I use Outlook for my EMail needs, and whenever I delete an EMail, it actually does not get removed. So as part of my weekly disk cleanup/maintenance/repair, and backup processing, I first run a Microsoft-supplied "method of having those "deleted" EMails removed permanently. The same could be true for other EMail programs, but I do not know much about them. If you let us know which EMail program you use, then maybe someone could help you with that.
Finally, you need to use a disk cleanup/maintenance/repair program (beyond Onyx). Disk Utility is "OK" for that, although to use it, it needs to be executed externally to your Mac. Two excellent commercial programs are TechTool Pro (I have that one), and Disk Warrior. Either of these do a super job regarding disk cleanup/maintenance/repairs.