Know how to put on and take off wheels. If your bike has quick release skewers (those leavers at the centre of your wheels), learn to tighten them correctly.
Gears are great because they essentially take the grunt out of hills. Before going on the road, try your gears, one changer will work the back gears and that's the one you will use the most. Most beginners tend to be afraid to take their hands off the handlebars to change gears so getting this down pat off the road is a really big help. Know how to make peddling easier and harder by changing up and down.
Modern bikes have really good brakes especially mountain bikes so be very careful when applying your front brake. When you first start cycling, it will pay to avoid using the front brake initially - start with the rear brake. Before you go out on the road squeeze each brake and note which is the front!
The best way to get a feel for your brakes is to ride at low speeds on grass or a flat road. Get a feel for how hard to pull and which lever is the front and which is the back. You will quickly learn the correct amount of pressure to apply. Be careful if you are using different or borrowed bikes as the brakes vary between bikes and might be a lot better or a lot worse!
The final thing to do before hitting the road is practice turning, the school ground is great for this so practice left and right hand turns.
Having done all of this, you should now have the confidence to get out on the road.
