Generally speaking, most schools take your highest score, even if that is not your most recent score. Also, generally speaking, most schools don't care if you take the test up to 3 times. If you know where you're going to apply, though, then you may want to check with the individual schools, as different schools can have different policies.
Re: getting better, we need some more information from you in order to help you. What have you done to study so far? What materials have you used? What are your strengths and weaknesses? The best way to answer that last question is to do the analysis described in this article:
http://www.beatthegmat.com/a/2009/09/23 ... tice-testsThen come back here and share your analysis with us. Also, if you didn't take the practice test that you analyze under full official conditions (including essays), then let us know exactly how you deviated from official conditions (even if it seems very minor).
Once we have that detail, we can give you some more specific advice about what to do!