They usually have 3 or 4 roads radiating from them, but this can be many more for the bigger roundabouts. Apart from mini roundabouts that are usually found in towns, they usually are well signed, both on the approach and at the entrance and exit junctions. This enables you to find which exit you require so that you leave the roundabout on the correct road. So as you approach look at the signs and count which exit you want. That is if your required road is the third exit then when on the roundabout you have to pass 2 exits before you leave the roundabout on the third.
Roundabouts can have one or more lanes around the roundabout.
This coupled with how far around you have to go before you exit the roundabout, determines which lane you should be in. Assuming it is a single lane roundabout, and you approach from the South and want the East exit, you therefore want the third exit. As you reach the roundabout. Traffic going around it, or traffic about to enter the roundabout from the east have the right of way, so you must stop and let them pass. That is the basics.