its not impossible to score well in a-levels if you haven't
taken geog in sec school, in fact, i'm living proof of that- got an
A in the a-level and i didnt take pure geog in sec school.
just study real hard, as in really hard. geog to me was like
taking 2 subjects.
dont worry if you dont understand most of the concepts at first,
over time it gets better; i flunked most of my common tests during
j1.
study smart too. what i did was to spot the topics, although its
not very advisable. a better way would be finding certain topics
which you like or understand the best, and make sure you really
know it well, while at the same time roughly know the others too.
so when doing the essay section, choose the that topic which you're
best at. eg, phy geog has 1) rocks and the like 2) atmosphere 3)
hydrology- i concentrated on 2 and 3, while roughly went through 1
so that i had enough knowledge for DRQ.
make sure you do your readings. almost everything in geog
requires real-life examples or events to be given either
descriptively or statistically. if you're abled enough but dont
roughly know the examples, you can try to smoke your way through
(NOT advisable tho).
print out the a-level geog syllabus. its extremely helpful cause
some schools like to cover extra matriel while others tend to skim
over certain topics. so by having the guidelines youll allow
youself to better know what to study.
one good thing about not taking geog in sec school is that you
wont be trained to write your essays point-based ( since o-lvl geog
was marked based on pts), so its easier to follow the LORMS way of
writing of the a-levels.
dont worry, just work hard, do your readings and have the guts
to consult your teachers if you have any queries, and youre well on
your way to doing well. 
