Started cycling last week. While riding cycle I am feeling like falling down on the bar, wrist gets pain, is it normal (my height is 5.6 and MTB cycle size is 18.5)?
"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.