Most importantly: WEAR A HELMET! Never go mountain biking without one!
Bike shorts with good padding, now also available as baggy shorts, but the clothing should not be too loose.
Shirts functional materials offer comfort when biking due to their good breathability.
Gloves long fingers protect your fingertips, also if you fall off!
Shoes special bike shoes or at least sturdy shoes! The sole should be fairly hard and ideally have a system binding for the pedals. This improves the transfer of muscle power and achieves a secure position on the pedals.
Bike glasses recommended as protection from insects, wind and stones.
Rain jacket and rain trousers if necessary (breathable): choose your clothing according to the weather! There are also windproof jackets and vests, winter shoes and many other clothing possibilities.