If you want to go in the suburbs, RER is definitly required.
It looks like to the metro way to travel.
First, He is most used by people living in the suburbs in the morning (6h30==>10H30) and in the afternoon (16H==>19H). I advise you to avoid to take it out of these hours, because they are few people in, and you could have problems. For exemple, one time we were with friends and an other band was smoking a joint... It's very rare to have problems, but I want you aware
So, watch your destination and look which direction you have to use. (Like with the subway). They are 4 lines but beware ! Sometimes the same line can be divided in 2 parts. So check the train you catch is the good one.
An exemple :
I think travel by RER can be quite hard if you don't know at all the suburbs, but it can be required if you want to visit monuments close to Paris (Like Versailles)
Ticket's price : I think from 5 Euros (7 A$). After, all depend where you want to go. The more it's far, the more you should pay I guess. When I go on Paris it's 7.5E (10A$), with the return ticket ; and I can use the subway all I want.
Map :