Is it a good idea to replace my iphone 4 display myself

There are lotta cracks… I really wanna replace it… What shuld I do?! How much will it cost?

Do you mean the glass lens? If your phone still can display, no need to replace the LCD, just replace the touch panel is ok, here is something you need in low price http://www.etradesupply.com/apple/iphone/apple-iphone-4-at-t.html

That is Absolutelyy a good idea, it won't cost you much especially compare to send it back to Apple and the most amazing thing is you can change a case in any color that you want.

You can get that screen from Muchbuy and search of tutorials on their blog.
Check this out: http://www.muchbuy.com/apple-ipad-ipod-replacement-parts/iphone-4-replacement.html