Your local police station might run cycle safety schemes that you can go on?
If they don't, the best thing to do is to practice by cycling on a Sunday when it's quiet first, and then work up to cycling in full traffic! Do practice using your arms to signal when you're turning left or right - this can be a bit unnerving the first few times you do it.
It doesn't take long to get used to it, even the roundabouts! Make sure you wear your helmet, though, and that you always have your lights and a high-visibility belt or jacket if you are going to be out after dusk - even if you plan to be back early, it's a good idea to always carry your lights with you. Don't leave detachable lights on the bike if you leave it in a public place as they can get stolen.