Question

how can i estimate in the first step of solving a division problem

Answer 1

Just guess how many times the first digit of the divisor will go into the first digit(s) of the dividend.

For example

253)820423

I'd guess 4 for the first digit of the quotient, since 2 goes into 8, 4 times.
In this case, it won't work, since 253*4 > 820, so we'd back off to 3:

3*253 = 759, so that would be how we'd get under way.

Hope that helps...