Being prepared while out on the mountain bike trail is an absolute must and a great rule of thumb is to always pack more mountain biking equipment the farther away from help you plan to be. There are a few important considerations you should also make regardless of the skill level of mountain biking that you plan to partake in.
The first consideration, for anyone planning on being active outdoors regardless of mountain biking or not, should be to have a well stocked first aid kit. You can purchase biker specific first aid kits, that are purposefully designed to fit nicely in a small pack, under the seat or even in saddlebags. A well stocked first aid kit will contain antiseptic, pain reliever, tensor bandages, band-aids, moleskin, and tweezers.
The second necessity is to have a mountain bike specific tool kit and there many options available as to the type of bike you have and the tools you want it to contain. The very basic tool kit should have the required tools needed to repair a flat tire and will fit nicely under your seat or in a saddlebag. Other tools to look for are an Allan key or small wrench, a chain breaker, a tire gauge, a Swiss army knife, and some strong tape.
Another highly recommended piece of mountain biking equipment to include is a spare mountain bike chain. Any experienced bikers know all to well how easy it is to bust a chain on the trail and the headache it can cause. Often a broken chain can be repaired but do not take the risk of being left without one.
Last but definitely not least is a portable air pump. These pumps come in numerous shapes and sizes and can be purchased specifically to fit your bike. Most can fit under the seat or in saddlebags, and often the repair kit includes one.
All these pieces of equipment are designed to be light weight and easily stored on your bike. Never risk traveling far from help without then because being prepared with the right mountain biking equipment can save you many hours of frustration.