First of all you need to be able to run MAMP in port 80. This is a "heat check" if you don't have any process jamming http ports. You can check it like this:
sudo lsof | grep LISTEN
If you do happen to have any process with something like this *:http (LISTEN), you are in trouble. Before with adventure check if it isn't MAMP itself (yeah, you should close that beforehand)
ps <pid of that process>
If you don't see MAMP, you are in good hands, I have just the thing for you: