How to clean up my iMac...

Purchased a new iMac today and do not want to carry forward all the unneeded files and folder, etc. buildup which is always passed down when transferring disk contents from old to new hardware. I have been a computer user for 20+ years and hate to think about all unless and corrupt data which must still be present from years and years ago.

Need to get rid of this mess...any ideas. Have and idea this is going to be a time consuming task but it is time to do something....