How to get the generator G point for any new curve?
You can't just change N. N is the number of elements in the finite group. You can pick a new (prime) P close to 2^256 (or however many bits you want) and get a resulting N.

1) G is a point in the group that when added to itself produces another point in the group. Hence it can generate all other points with repeated additions (generator). If the number of elements in the group is prime then every point has this property and can be used as a generator (via Lagrange's Theorem).

2) Just use the curve equation. y^2 = x^3 + 7 => y = modsqrt(x^3 + 7) (everything is mod P)