1. I have this problem with Airbus A321 and A380 i.e. if I am playing a free flight and plane is running on runway then it auto takes off. I am not facing this problem with other planes. So basically I am on runway ready to go, I press F4 to full throttle, when it reaches speed around 140, 150 then it takes off near the end of runway. Why is that?

 

2. When I play free flight then my plane always starts at the runway from where it will take off. I don't want that. I want to taxi the plane to the runway and then take off from there. For e.g. if I am supposed to take off from runway 7R then at the start of free flight plane will already be there so I just have to throttle and go. How to taxi to runway?

For #1, at 150 knots the airplane will probably just start lifting off on its own because the wing is generating lift, unless you push forward on the stick to hold it on to the runway.

 

To start at a parking space or a gate, you click the button for current location (or departure location in the flight planner). and choose a starting location using the choose runway/starting location drop-down list shown here:

Airbuses, in general, tend to do that.  I would suppose that you have a notch or 2 of flaps set for

 

your takeoff so that will increase the lift on your wings.  Do you also want to start your own engines,

 

turn on your lights, and stuff like that when you start your flight at the gate?  Google search FSX

 

Cold and dark startup.