If anyone can help me with a simple conditional formatting I'd really appreciate it. I'm trying to compare two lists and highlight cells in column 1 that also appear in the list on sheet 2. Tried google and for some reason it is not working.

Let's say list 1 is in column A of Sheet1 and list list 2 is in column A of Sheet2. Highlight list 1 > Conditional Formatting > New Rule > click the last option (Use a formula...), then enter the following into the Formula box:

=COUNTIF(Sheet2!$A:$A,A1)>=1

Then you can click format to choose your desired formatting.

Keep in mind that this version of the formula will be slow if you apply it to massive lists. There are ways to make it more efficient if necessary.