well, you donīt realy need money to survive in SL. But having this full featured car, owning land - making money with your bussiness are reasons where money is needed.
Basicly what you realy need is a verified operational paypal ( www.paypal.com ) account, and it must be connected to your SL-membership-account. that doesnīt mean that you need a premium membership.
In SL we use Linden Dollar (L$) wich is a free flowing, traded currency. The Exchange rate to US$ or is based on how many want to buy or sell, and not only Linden Labs are trading. (Btw, there are even people running in-world bank systemens, paying you interest for your savings. Have a look at the ATM at http://slurl.com/secondlife/Daydream%20Islands%20CTR/36/164/23/.
Right now, i know 3 main monex-exchangers. Be carefull with private trade over ebay and so.
LindeX - the Linden Labs built in system. You can use your web-account-menu, the builtin feature in the client, or a Lindex ATM like http://slurl.com/secondlife/Daydream%20Islands%20CTR/36/164/23/.
slexchange ( www.slexchange.com ) has a money exchange - nice overview over whats going on in meanings of bid and request. They even have in-wold ATMs.
ACS ( http://www.anshechung.com/ ) is although running a web-based money exchange.