I am beginner and want to make a quadcopter .After doing some research,watching videos and reading articles I am now making a mini quadopter using arduino nano.I will be using coreless DC motors.But I am confused a lot because I cant find full tutorial anywhere. Here are some of my questions:
1) Which sensors are really necessary for my quadcopter ?
2) Can the motors be directly connected to arduino nano (flight controller) ? if no then how can i connect them (i guess using l298 modules)?
3) Which would be better 2.4 GHz NRF24L01 or 433 MHz wireless rf module ?
4) How to select right motors and modules according to battery output ?
I am also making my own transmitter using arduino nano and two joysticks for throttle and YPR values . So, do I have to program transmitter for sending signals to my drone ? If yes, how can I program it ?
 

Hi Ayush, welcome to the community

If you are a beginner and this is your first time building a quadcopter, then I would strongly suggest against taking the DIY route, unless you have an underlying purpose, probably a college project or something.
If that is the case, then also, do not take the DIY route on the Flight Controller unless you have have command over embedded systems and programming arduino devices, because your chances of getting success, well let's be honest, are quit slim.

That being said, and my apologies for scaring you, let's get back to business. I'll answer your questions one at a time to the best of my ability.

1) Which sensors are really necessary for my quadcopter ?
A: For the most basic version of a flight controller, you need to measure two things, what is the change in the angle of the quad in all the axes relative to normal plane, and how fast that change is occurring. Basically angular displacement and angular acceleration. You can measure displacement using a piezoelectric gyro and rate of change of the angle using an accelerometer. These days you get 6DOF or 10DOF IMU sensors which do the job quite well, some examples are :

http://f4k3url/2mdb2kI
http://f4k3url/2nbdwET , you can find others too, pick one based on your choice

2) Can the motors be directly connected to arduino nano (flight controller) ? if no then how can i connect them (i guess using l298 modules)?
A: no, you cannot connect the coreless motors directly to arduino, it will be dead in no time. Arduino board shells out a maximum of 150mA of current on the PWM pins, and these motors (am assuming 8520 coreless ) consume somewhere between 750mA to 1.2A current. You have to use a motor driver definitely, L293d might work, but am not sure if it can handle the current draw of two motors simultaneously, I'd suggest you use high drain Mosfets and create your own H-bridge.

3) Which would be better 2.4 GHz NRF24L01 or 433 MHz wireless rf module ?
A: NRF24L01 hands down, better range, better connectivity. 433 MHz modules are piece of crap, keep them to your mini projects only.

4) How to select right motors and modules according to battery output ?
A: Its the other way round actually, you don't select the load according to the battery, you select battery according to the load. In this case, a single cell LiPo would do the job, check this out http://f4k3url/2lOAOjG . And please, do not attempt making your own battery using mobile cells, its futile.

Creating the TxRx system is a whole different ball game pal, if you get any success in this, do ping me, i'll be more than interested.

Cheers

And one more thing, if all that looks like too much of an effort, then go for this

http://www.quadkopters.com/product/arf-kit-rtf-kit-and-frames/qk100-arf-indoor-kit/

It'll make your life easier

Hello Ayush,
Based on your questions, I can safely assume that you do not have an expertise on making complex projects. Making a quadcopter from scratch is a very tough one, and I also echo with Bilal that you shouldn't go the scratch-DIY route. Follow the link Bilal has given, you'll find some ready-to-fly quadcopters there.

Regards

Thanks bilal and dipanjan.I think I can manage to make quad but it would be difficult to make the transmitter.  Can you pls tell me what are the complexities in making a transmitter ?

Actually a simple transmitter might not be that hard to make.you need two nrf24l01+ modules talking to each other ,one preferable lna+pa version .tge complexities arrive when adding features to transmitter like a good display.for that 128*64 lcd is preferable on which you can see the stick positions etc .last but not the least. You will need to add digital trims.i cant tell you about how we can add digital trims(i am confised whether to use momentary tactile button or something else.)also forget wrriting in arduino c++ and try to use avr embedded c if you are using atmega.a usb oscilloscpe if possible should be used to see the pwm /ppm output and check for any errors.but given that this is your first quad i would take an easy way and then climb up the hill

I really want to make it myself  
What I have seen is that people make quad using brushess motors , ESCs some flight controller like f3 or multiwii and then buy a transmitter to control it. But I want to make my own transmitter. And to keep its cost low I decided yo use coreless motors which are lot cheaper and doesn't need ESC (by the way were you talking abut the IRF540 mosfet for connecting motors).Thanks for helping !