Gradual exposure

Start riding on the roads you're familiar with, when there's not too much busy traffic and where the junctions aren't too complex. You can work up to busier and bigger roads as your skill and confidence grows.

Don't worry about walking across junctions you find diffficult. it's better to walk a little of a journey than to leave your bike at home.

Take a little time to watch the traffic at places where you find problems. Which lane of traffic goes where? How do other people on bikes manage? You can learn from watching both the accomplished and the incompetent. You'll soon make progress and be riding where you used to walk!