What you're asking for is still slightly unclear. Are you asking for help in converting
audio or
video files? The answers are different.
- Audio (wma to mp3 or AAC) -- iTunes has this ability built in
in Windows only, and for unprotected WM files only. The Mac version of iTunes does not have this feature. One easy way to convert these files is to use iTunes for Windows, if you can access it. Just open iTunes, make sure that in preferences -> advanced, the import quality is what you want (AAC or MP3 and the appropriate bit rate) and then import the file into Windows iTunes. iTunes will automatically make a converted copy that you can then migrate to your Mac.
If you don't have access to a Win PC or don't want to do this, then
EasyWMA is one option on the Mac (not free). There are a few other options, discussed above. I think the Windows iTunes one is the most hassle-free.
- Video -- this is a different matter entirely. There are a variety of converters, it would probably help to know the codecs involved, though.