i left mine fairly late, and did a study of local creek. my focus question was 'asses the human impact on greystanes creek.' pretty good marks, 19/20 for essay, lost a couple in the presentation-im a terrible presenter.
my adivce for anyone doing this-if your on here for help, chances are you left it late. pick something local, and pick a topic that isnt too narrow. my mate was going to do a sgp based off water quality tests, but it ended up really hard to expand on. also i can't stress enough the need for stats. both first-hand results such as surveys, water tests, and the like, as well as photos to back them up, but also data from secondary sources, such as planning maps, census data, etc etc.
other than that-make a schedule, stick to it. if you plan to do field work on monday, do it monday. if you decide to do it the next day you wont.
above all-have fun. if you're doin it you are still in year 11. the hard stuff doesnt start till next year.