I'm a recent UBC Graduate, and I did transfer faculties too - so I think I can help.
Transferring within faculties is usually no big deal. The advising offices are okay, not the best (they do their best with the huge volume of students that they have).
What I would do is go to Science Advising BEFORE you start your classes. I transferred from Science to Engineering.
The thing is, you cannot transfer between faculties in the middle of the school year. Your request to transfer must be made at the end of the school year, and they usually let you know in the Summer (depending on your grades etc). What I recommend you do is that you check out the typical science course load: First-Year Options - Bachelor of Science - The Faculty of Science - Faculties, Colleges, and Schools - Vancouver Calendar 2010/11 - UBC Student Services
I suggest that you take the first year science courses. UBC lets you choose your own time-table, so plan ahead (they will give you a registration date ahead of time). Make sure you sign up for the core science courses, Math 101, Phys 101/102, etc. Since you haven't had Chemistry, I don't think you should take Chem 121/123, they have Chem 111/113 (I believe) that you can take. Instead of wasting a full year taking an arts curriculum and then having to go back to First Year science in your second year, I would highly recommend you take all the first year science courses, even though you are in arts (I mean, who really cares, you'll be transferring into Science anyways). That way, when you get into Science, you can continue from 2nd year Science. It'll save you time, and if you are international, it'll save you a lot of money.
In any case, Science Advising should be able to give you more info on that.
Make sure that you keep your average above 70% in your first year, and you should have no problem getting into UBC Science.