This habit may begin as soon as you switch from a crib to a toddler bed. This is a common problem and is correctable. You must stay consistent and not accidentally reward the activity by letting them sleep with you or giving them lots of attention. This approach will quickly teach your child that getting out of bed is not worth while and should break the habit. Begin by putting you child to bed using your normal routine (example: reading a book then tucking in).
Leave the room and reassure them you are close. The FIRST or SECOND time your child gets out of bed take them back as soon as you see them and tuck them in again and stay for a moment. Make sure you are not staying long enough for them to fall asleep. Every time after the first or second time take them back and leave after putting them back in bed. Do not stay in the room. You may do this 10 plus times the first week, but soon after the first week the number of times your toddler will get out of bed will lower dramatically until they learn to stay in bed.