Sorry. Sampuk a bit.

How to know when is the time to do rotation?

Sent my 1 year old car with 50k mileage to SC to rotate, SC said no need.

You replied your question:

So i just did my 10k km service..a normal front wheel drive sedan car, and I wish to do tyre rotation, internet says I should do forward cross right?

Also what is the right way to do it? Do i just take off the rim and replace the position(that will change the direction right?) , or i actually have to take off the tyre individually and change it without changing the rim location? The tyres are non directional tyres.

Do i need to rebalance the wheel?

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Changing wheel from 1 side to another AND the reversing of the rotation is the point of doing tyre rotation.

If you have budget, then why not re-balance it to REDUCE ROTATIONAL VIBRATION at high speeds?

But can see the front tyres thinning whereas the rear are still like new. 

+1

Think about it, what if rotating tires actually wears them out faster?

A tire used to roll forward then gets rolled backwards, meaning it has to be broken in again.
A straight line rear tire from the rear now gets rolled backwards, and driven in the case of FWD tire just gets worn out faster than it should be.

A tire with properly worn in pattern provides better weight distribution during roll, and grip. Imagine if we can change our shoe soles from left to right, we would have to break in the soles again, how that affects treadwear and grip?

One thing worth try, is to rotate only one side front back. Or, cross rotate front left and rear right. Leave the other set alone. See how that goes?

What you say about faster wear after rotating tyres or change 'position' or 'contacting surface profile' or multiple times of 'broken-in' makes perfect sense , IMO  . Unfortunately this 'negative' phenomenon is being kept aside or hidden from consumers.