Hate to say it, but practicing them by drilling them over and over is probably the best way. Even though you haven't done the textbook problems in class, teachers can always throw a curve-ball and ask you weirdo stuff.

Systems of linear equations can be solved various ways. On you probably don't need to know is via matrices

Two other ways that you probably do need to know, being in Algebra I are elimination and substitution.

For substitution, solve one equation in terms of one variable, then substitute that into the other equation wherever that variable shows up.

For elimination, multiply by certain factors (can be negative) so that when you add the two (or three) equations together, one of the terms cancels out.

The quadratic equation will be your best friend as long as you make sure to memorize it exactly.
x = (-b ± sqrt(b<sup>2</sup>-4ac))/2a
where a is the coefficient of the x<sup>2</sup> term, b the x term, and c the constant (no x).

If you want some real-time help, drop by #eoff sometime, and I'm sure someone (愛 or myself, or others) will be happy to help you out.