New to cycling, wrist pain while cycling
"wrist gets pain"
The wrists need training, so some pain is understandable for a first time rider.

"feeling like falling down on the bar"
This is the normal position on a race bike. It is NOT normal position on a MTB. You probably need the handlebar at a higher height, or closer to you.

Also, looking at a "mountain bike size chart", it might be that the bike is too large for you, you'd probably need M size, or 16-17 (18.5 is XL).
This can be partially alleviated using a short stem (the handlebar as close to you as possible), but a smaller size bike will be better.
You could also shift your saddle forward some, but that changes the "seat above the pedals" distance, and brings with it other problems.

EDIT: At your height, you're just a bit over "S" size recommendations, so 16" frame would be better.