What to Wear for Mountain Biking

You don’t need cycling specific gear when you’re starting out, although proper cycling clothing will be a lot more comfortable in the long run.
The basics are: a cycling helmet, tracksuit, cycling jersey or wicking
t-shirt and a pair of trainers with grippy soles.
You should also bring a wind/waterproof jacket at all times of year, both summer and winter – as well as keeping you dry it’ll protect you from the wind on exposed hillsides.

Other things you might need are:

padded cycling shorts or tights – after a few hours in the saddle you’ll really appreciate a good pair of shorts
cycling gloves – as well a providing comfort when holding the bars a pair of gloves will protect your hands in the event of a crash. There are long and short fingered ones depending on your preferences.
cycling shoes – proper cycling shoes will have stiff soles to make pedalling easier
clipless pedals – some riders prefer flat pedals but clipless pedals and shoes can provide more power when climbing and more security when descending
sunglasses – these can protect your eyes from mud and debris thrown up by your wheels