There are a few essentials which you should have with you on every ride:
Water – at least one drink bottle full
Food – you use up a lot of energy while mountain biking so it’s important to refuel regularly. Bring whatever you like to eat. Fig Rolls are always popular.
Spare tube (or two) – You’re sure to get a puncture eventually so it’s wise to have a spare tube with you. It’s faster to change a tube then to patch an old one (you can patch it at home later)
Pump – You won’t get far if you can’t inflate your tyre after a puncture. Don’t assume someone else will have one
Basic tools – at a minimum you should have tyre levers and allen keys for the bolts on your bike.