installing mamp is pretty strait forward, download the installer form here:
http://www.mamp.info/en/
if you are familiar with XAMPP on windows, it's not really that different.
in XAMPP, it would create the site root as:
c:\xampp\htdocs
which mapped to the site address:
http://localhost
for each DW site you would create a subfolder of:
c:\xampp\htdocs
for example
c:\xampp\htdocs\site1
and adjust the web URL appropriatly:
http://localhost/site1
Same strategey applies with MAMP, the only difference is where the htdocs folder gets created.
in MAMP, the htdocs folder gets treated at:
Applications:Mamp;htdocs
create the subfolder for the site there:
Applications:Mamp;htdocs:site1
and the same web URL will link to it:
http://localhost/site1