I am new in RC modeling. i would like to build my electric flyer. I want to know what stuff to buy so that i don't need to spend again for same stuff.

If I want to buy ESC what AMP should I buy? I want to start with small motor. I can buy big motor later. So do I need to buy small ESC or if I buy big ESC it will work with small motor without burning it.

You can get a 50A ESC, it will work for a decent range of motors.

It will work with small motors, but weight and size could be an issue if you're building micro models using micro motors.

Also, make sure your ESC can handle the battery packs you intend to use. Some ESCs are limited to 2s (7.4v), 3s (11.1v) and so on.

welcome to rc world

u could definitely go for a high amp ESC without any problem . the current it will give will depend only on the load that is connected to its output, so a small amp motor will cause the ESC to give small amps and when u connect a large motor its output will increase accordingly.
but of course large amps ESC will be heavier, so u have to decide , whether its worthy of your investment or not.

what anjanbabu has said is correct.

Start with your model weight. build it first and check the weight, lets says if it comes around 100gms, add 200gms of battery and 30gms of ESC and other tit bits, and a motor for 50gms, this is your AUW. now select a motor that can handle the AUW. usually the recommended model weight will be given there in the motor specs, then select the ESC as per the motor recommendation. lets say if a motor max amps is 25A, then buy a 30A ESC for safer side. then buy a battery with C that can handle the amps rating. This is the tree that i follow for selecting my power train.

Ashok.P