and dont be to disheartened by your first few goes, it does take practice, the more you do the better you get.
the key to good welding is preparation; you cant weld rust, so grind it off, if the metal left is to thin, dont bother with it, cut back until you've got enough, lets face it, tissue paper thin metal wont weld nice ( with mig) and it isnt strong enough anyway.
and last thing buy a decent grinder its worth it.