Okay, assuming you have already bought the prepaid phone...
the card should have the instructions right there. You may have to activate the phone first. This should take about ten minutes at most.
The card has a 12 or 16 digit number, depending. You enter those into the phone and it will add the requisite amount to your phone.
$10 will get you 30 minutes of call time. $25 will give you 150 minutes. Definitely worth it to not go cheap unless you really, really have to.
I love my prepaid T-mobile phone. I will never go back to a plan again -- not even for an iPhone. And AT&T's prepaid plan sucks.
Let me know how it goes