In general, your statement of purpose (SOP) is a 1-2 page typed description that addresses three areas: (1) Your educational experiences and how they have led to your interest in graduate study in the geological sciences; (2) a description of any scientific research you have conducted (be specific and include field experience, lab/analytical work, a summary of your findings); and (3) a description of your research interests for graduate school and your career goals. A well-crafted statement of purpose is specific about what you want to study and the geology faculty member(s) you are interested in working with. Do your research about the faculty and department to which you are applying and tailor each of your SOPs to that faculty member/department. You do not need to know exactly which one faculty member you want to work with - it's OK to include two or three names (maximum) if they are working in related/complementary fields (but don't include, for example, a geophysicist, a geomorphologist, and a hydrologist - that would demonstrate that you have no idea what you want...). Nor do you need to know exactly which project you want to work on - be as specific as possible about your interests ("extensional terranes", "metamorphic core complexes", "ultrahigh-pressure metamorphism") and certainly include a description of how your research experience has prepared you to tackle your proposed research. Look through the websites of potential graduate advisors to see which projects are active. You can also search the National Science Foundation website to look for active research grants for those prospective advisors (although not all research funding comes through the NSF, that is a major source of funding in the Earth sciences).
This web site and this link have good descriptions of the basics of the SOP (even though these are intended for graduate programs in psychology) and some things to avoid. Whatever you do, don't start your SOP off with the all-too-common "I have loved geology since I was just a kid starting out with my first rock collection" sort of statement. Gag me. Just start with your college experience, or perhaps one notable trip you took where you learned about geology. Be matter of fact, as specific as possible, concise, and use the SOP as a place to elaborate on things only briefly included in your CV (like your research experience) or to discuss things that don't appear elsewhere in your application and that are relevant to your application (e.g., if you need to explain poor GRE scores or a D in a chemistry class in your freshman year). Stay positive and don't make excuses.
Spell check, proofread, and have someone else read your statement and give you feedback before submitting it (a grad student, a faculty advisor, swap SOPs with other students). The rest of your academic record can't be changed at this stage, and you can't control what your letter writers say, but you CAN write an excellent SOP.
*GRE scores are no longer required to apply to the MS program in Geosciences at SF State.