I have tried doing them but i cant perform them without holding something
is it because my legs too weak for balance?

I saw on youtube one guy doing pistol squat on a dip bar i was very confused lol he is surely beast balance and strength!!

I also need help with building leg routine because I never actually trained legs I want to transform my legs to monster big huge fast balance strong legs.

how i achieve this?
or if you achieved how ?

You need to work your way up to pistols, they're difficult because of the balance and strength required for them. You can do squats, lunges, calf raises, pistol negatives, car pushes (really killer to finish a leg workout. Put the car in neutral, have someone steer, and push that baby down the block focusing on using your leg muscles.) Doing sprints will help condition your legs as well. I'm sure there are other exercises for your legs that someone else will post.

Here's a pistol tutorial that Daniel made:

yes, your legs are too weak for them. like fkncody said, you gotta work up to it. if you rush into it, you'll have poor form and may likely hurt yourself.

Pistols,Pistols with jumping 4 height & Distance, weighted squats

Glute Ham Raises (GHR)

Calf Raises

i know this will sound strange, even harder, but if u hold a 5kg weight out in front of u in your hands as u go down this will act as a counter balance and help you get those pistols out. Just make sure you keep your heal on the ground flat at all times.
If 5kg is to heavy, find something that is a little lighter, but still able to push back up from that pistol.

I found this method helped me out the most to get that balance and then also get the strength needed.