i am looking for free software to clean up my iMac

Welcome to Apple Support Communities.

Do NOT ever install any “anti-virus”, “cleaners”, “optimizers”, etc. Mac computers do not need any sort of these Apps. The only thing they are good at is creating false marketing promises and taking consumers' money. They are all scams. In fact, anti-virus and other cleaners cause performance issues, security issues, and make macOS appear buggy. Your Mac is far worse with these types of Apps installed.

Mac computers have protection built-in, which is the best protection for your Mac. For specifics on macOS Security, see: macOS - Security - Apple and Protecting against malware in macOS - Apple Support. Mac computers also have built-in storage recommendations: Optimize storage space on your Mac - Apple Support. You can even use the built-in macOS Firewall: Block connections to your Mac with a firewall - Apple Support. Other than that, all you should do is restart your Mac approximately once a week and keep macOS up to date: Update macOS on Mac - Apple Support.

Jack

+1 - Run EtreCheck

EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

You have the correct concepts in the two responses, but what exactly do you believe needs cleaning up?

Is you disk full and you need to reclaim space?

Do you have a malware incursion?

Are there too many duplicates files and you want to thin the herd?

What exactly is your goal; your need?

Get Etrecheck Pro, run it and post the report here using the Additional Text icon. EtreCheck will look at you system and note problem areas.

“Cleaning up” will likely not get you any significant speed increases. Slowness can be caused by a number of things such as a slow drive, too many startup apps, certain apps and etc.

You do not want to use any so called "cleaning" app, free or paid.