Initially, do not give your child the freedom to get out of bed without your permission. After his naps or in the morning, have him call out to you before getting out of bed. Finally, when making the transition, buy or build a side rail. Children move during their sleep much more than adults. Side rails are as much for the parents' peace of mind as for the child's safety.
Children are wonderful decoders.
By twelve to fourteen months, he'll understand enough of the words and intonations to figure out exactly what you are telling him. Children are wonderful imitators too. This gives your child a chance to pair words with concepts. Even though he will not understand all the words at first, what a great exposure to the world you are giving him. When you go to the grocery store, talk about what you are getting, where you are going next, things you see in the aisles.