Start by putting a litter box in the cage. Use rabbit safe litter such as wood or paper pellets. It is best to use something different than what you use in the rest of the cage. It can also be a good idea to put some hay in or near the litter box so your rabbit ca neat the hay while in the box as rabbits like to pee and poop while eating.
If you rabbit is using a corner to pee and poop, but the litter box there. If no corner is used, then just pick a corner to put the box in.
Not all rabbits will litter train right away. Young rabbits and intact rabbits can be harder, but it is still possible. Young rabbits can have trouble as they can't really control it too much yet. Intact rabbits tend to want to mark more.
It is a good idea to not use shavings or other similar bedding in the cage, you can use a towel or some fleece or something similar. This makes it easier to pick up poops to put them into the litter box. For pee, you do want to be able to clean it up with a paper towel and put that in the litter box. You may want to have a few blankets so you can keep the one in the cage clean.
Try to keep the litter box clean but not too clean. Scoop out the poops and really wet litter daily, but leave some in so the smells as still there. As you rabbit gets better using the litter box, you can clean it better and scrub it out.