This page will explain how to purchase money on the Lindex. There doesn't actually seem to be any kind of tutorial on the official wiki. It shouldn't really be needed, as it's straightforward, but it's good for completeness.
LindeX is the exchange system for Linden Dollars and real world currency.
Purchasing Linden Dollars through LindeX is hopefully straight-forward enough that it doesn't require a tutorial, but if you're having problems, this should answer your questions.
This tutorial only covers purchasing Linden Dollars.
Buying fees Selling fees But there's no way around it. Be very, very wary of someone selling or buying Lindens with less fees than the LindeX.
A screenshot walkthrough that explains all the different things about buying.