how do you find p when trying to find the focus for parabolas

It depends on what kind of information you are given, and what kind of parabola (vertical or horizontal) you have.

If you have a vertical parabola you can get it to be in the the form (x - h)² = 4p(y - k) by completing the square. Then the vertex is (h, k) and the focus is (h, k + p). Since it's a vertical parabola, we change the y-coordinate of the vertex.

If you have a horizontal parabola, of the form (y - k)²  = 4p(x - h), vertex is (h, k) so focus is (h + p, k). (Since parabola is horizontal, the focus is related to the x-coordinate of the vertex.)

That's a super general answer, but I hope it helps for now.