Specialized Mountain Bike Bags

Going mountain biking for the first time. Any advice?
Is there anything special I should carry in my bag?
Any advice in particular regarding clothing? I have no specialized clothing and the local shop is closed so it’s a no go.
Got a helmet that I used to use for quads so i’m sure it’ll suffice for bike riding.
Watch the Science channel tomorrow at 12 pm eastern time Sat 3-20.
I like to carry a water bottle.
Downhill Domination T-Bag Specialized trick

