This is one of the repairs that you should not handle unless you totally know what you are doing. One of the most critical things to getting an alternator put on correctly is getting just enough tension to hold the belt in place. You don't want it too tight because the stress could ruin your alternator. You don't want it too loose because you'll just throw the belt.
If you have limited space to work on anyway and little experience in getting that tension just right, the chances of botching the job are great. If in doubt, go to a professional. If short on funds, pay the mechanic down the street a six-pack to come help.