The CS GRE has been eliminated. It’s still possible to take the Math GRE, but that would only help if you’re interested in a theoretical field like algorithms or FLAT. Even then, a great Math Subject GRE score won’t mean nearly as much as research experience or LORs. It’s also an extremely difficult test and you should only take it if you were a Math major. Even if you became a Calculus expert (and most CS students I know do well in Calc 1 from high school knowledge, then take a B in Calc 2 because they find it too hard or just don’t care), and even if you took linear algebra (sometimes recommended for CS), discrete math, and probability, you probably wouldn’t be able to answer much more than half the questions. Unless you’re planning on learning subjects like topology, real analysis, complex analysis, and linear and abstract algebra very thoroughly, you’d be better off skipping the subject test. Pretty much everyone taking it is someone applying to Math Ph.D. programs.
You would probably need to outscore 80% of the test-takers for it to make a positive difference (for top schools – for lower-tier schools, it’d probably be easier to get in and the test score wouldn’t make much of a difference anyway). Also, GRE results are only good for 5 years. Don’t take it any earlier than your junior year.
Look for research experience, including summer REUs.