Between the ages of 18 months and 3 ½ years, you'll find your child becoming too big for their cot and is ready to move to a bed. Initially, the main point of this transition is for safety. Your toddler may begin trying to climb out of the cot during the night, moving them into a bed will ensure they don't injure themselves in doing so. If they do manage to jump out, don't panic! This behaviour is normal, so try to work with the cot as best as you can until you can get a bed.
When transitioning from cot to bed, it helps to try and keep their nighttime routine consistent while they become accustomed to their new sleeping arrangements. However, this doesn't mean boring—try including bedtime stories, and toys, as a way of making the routine enjoyable. Your child is less likely to focus on the loss of their cot, which has become their space, and more likely to enjoy the change. Additionally, you might like to let your child personalise their new sleeping space by picking out a duvet cover and sheet set of their own.