Just a few pointers. For equations use == instead of =, and then you can solve for y this way:
Solve[x^2 + y^2 == 25, y]
Look up the documentation for Solve. You can make a plot of you two-variable equation altogether with ContourPlot:
ContourPlot[x^2 + y^2 == 25, {x, -6, 6}, {y, -6, 6}]
To find center and radius one way is to use SolveAlways:
SolveAlways[{x^2 + y^2 - 25 == (x - x0)^2 + (y - y0)^2 - r^2}, {x, y}]
