KK 2.0 stability

I am building a quad copter.. with the following config

Controller - KK2.0
ARDUINO MEGA 2560 as a receiver for throttle, rudder, aileron and elevation.
Esp8266 wifi to Arduino
Android app connect to esp soft ap as a transmitter

Even when I use a transmitter instead of an Android app the drone drifts even before lifting off from the ground. What am I doing here ? What should I tweak to get the drone to lift vertically instead of drifting ?

This is probably a better question for people who know quadcopters. But if you still want to ask here, you're going to have to provide some details. I'm not sure what you think we can do from that vague description of the problem. You'll also need to post or at least link to the code. It's not safe to assume that the people here know about everything that's ever been done with an Arduino. You should probably also describe your hardware. You've basically given us nothing to go on other than the name of the flight controller and a description that it is "drifting". I'm sure you can understand that it is going to take a little more than that to figure it out.

I really do think you'd have better luck on a quad forum where people know a little more about what you are doing and you might even find someone else using that same flight controller.

I'm sorry it's my first time using forums and need a little more exp, the code is just a simple 2 steps
1.if serial data is available - read it - parse it to get throttle
2. Write it to the throttle pin using writeMicroseconds.

I can see the throttle increasing in the receiver test in kk board. But the drone drifts to one side with the minimum of thrust.

I have set self levelling to on.

If this is not the relevant forum, please by all means direct me to one .

OK, so you completely ignored the things I said you should provide, hardware description, details about the problem, links to the code. I'm not going to waste anymore time with you if I can't get the information I ask for.

Prominence:
If this is not the relevant forum, please by all means direct me to one .

I don't fly quadcopters so I don't know any quadcopter forums. It seems like that would be so ridiculously easy to find with Google. Do I you really need help doing that?

HobbyKing site has a Q&A section at the bottom of the sales listing for the flight controller.

Items to think about: do you have all the props turning in the same direction (BAD for stability)?
does the KK2 software have PID loops incorporated (last time I used a Kaptain K controller, they did NOT have PID)?

Your problem relates to uneven thrust, torque, and weight distribution. Heli’s have trims to make up for those effects. in quads, you do it with nimble fingers on the controls (good luck with that using WiFi, too slow), or PID loops to vary thrusts proportional to variation from the desitred attitude (level flight).
Heavier birds are less suseptable to those variables.