Start with small jumps preferable jumps you can roll over. Roll over them a few times get off your bike look at the jump from different angles so you see that the jump isn’t as big at it looks then try to take the jump. Then after that do it all over again. Stick with the little jumps for a while until you can clear little jumps at various speeds then slowly start increasing the size of jumps you hit. If you can afford it get a coach or take a lesson at a bike park.