The second thing you will want is Padding. You should start with at least Knee Pads. It may not seem like much, but you won't regret them when you take a hard wipeout. I personally went a whole summer without Pads, I was lucky enough to not take a big wipeout. When I moved on to bigger obstacles, I started wearing knee and elbow pads. I soon moved up to a full face helmets. That being said, I reccomend elbow pads aswell. They are worth buying to avoid uneccessary pain.

The last thing you want is without a doubt, a bike. Unfortunately Mountain Bikes aren't cheap. Due to their advanced design, small parts, and suspension. For this reason, I advise you to buy a good beginner hardtail bike (A bike without back suspension). They are arguably just as fun as a full suspension bike and can do almost everything that a full suspension bike can. I have personally been riding a Hardtail for quite a while now and they are very enjoyable. They also teach you important skills like absorbtion of impact in the legs and cornering.