How can I clean up my user Library folder?
Your Library folder contains several things which may be quite large: all your mails (including attachments), various caches, databases of some applications (e.g. Evernote) etc., so the big size might be ok (or even desired!).

To identify the big stuff you can open Terminal.app and run

cd ~/Library
du -sm * | sort -n


which will show you the largest directories at the end. Use cd again to drill further done and repeat the du command.

PS: Just for reference: My Library folder currently holds about 16 GB of Evernote data, 4 GB Developer docs from Xcode, 1 GB mail and 1.5 GB iOS software updates.