Once you have learnt how to approach roundabouts safely, the next challenge you will face is being able to judge when you can move into the traffic flow.
Don't be worried if you find this difficult at first, most people do. However there is a way you can teach yourself the technique without any risk.
LOOKING EARLY FOR SPACE...
The first thing to realise is that you are not really looking for cars at a roundabout you are looking for space. If as you approach you see a vehicle coming from the right, don't immediately think "I've got to stop" look beyond the car to see if there is space behind it. This way you can adjust your speed so that the vehicle passes as you get to the roundabout and you can move in to the space after it. If there is a line of traffic then you will know that you have to stop. The key though is to start looking early at the traffic flow at a roundabout. If you leave it to the last few metres it's more likely to go wrong.
TRY IT FROM THE PASSENGER SEAT...
When you are in the passenger seat with parents or friends, practice looking at the traffic and trying to judge where each vehicle is going. But because you are in the passenger seat while someone else is driving there is no risk if you get it wrong. As you practice more you will get more confident that you can spot spaces in the traffic and identify opportunities to move into traffic. You should then be able to take this confidence into your own approaches into roundabouts.
So to recap,
Look early as you approach the roundabout looking for spaces in the traffic.
If possible adjust your speed to be able to use those spaces.
If there is no space then stop, and look for other spaces to move into.
Practice by sitting with parents and looking at the traffic flows.