I'm new to physics on scratch, can somebody help me

You could draw your pendulum costume with the pivot point (ie. the top) of the pendulum at the centre of the pendulum costume.

Then you could use:

forever
point towards [mouse-pointer v]
end

what about the gravity
i'm stuck with making the gravity

You haven't any pendulum project shared, i think.

https://scratch.mit.edu/projects/382464494/ may help

As per @Scratch-Minion, draw the pendulum with the top of the pendulum at the centre of the costume.
Draw the rest of it horizontally to the right, rather than down (works better with the script below).
Then this may be along the lines of what you want:

forever
if <mouse down?> then 
set [angular speed v] to [0]
point towards [mouse pointer v]
else
change [angular speed v] by ([sin v] of (direction))
turn cw (angular speed) degrees
end
end