Generally the mics built into the laptops aren't great, but they should still work.
Go to Start > (All) Programs > Accessories > Entertainment > Volume Control
Do you see a mic volume control from there? If not, go to Options > Properties and put a check next to Microphone in the box below (scroll down). It should now appear.
Now put a check next to Options > Advanced Controls if there isn't already one. Click the Advanced button and put a check next to Mic Boost. Click OK.