I have just one question, why are you wanting to keep it a secret?
You may not show for a bit, but to hide the bump, baggy shirts, or thoes pretty little tops that kinda drop from the bust (if you know what i mean? they tend to make you look skinnier too :p)
with drinking, just drink coke, they dont need to know theres nothing else in there :p or grab a bottle from home (like a cruiser or something), fill it with similar coloured softdrink, no one will ever know :p
no idea about the sports thing, you can say your not felling well for a couple of times, but that may get a little suspect….
Also nausea is a bit of a give away, so try and eat small meals regulally so that you may not feel as sicky, and try and have a protein snack before bed.
another thing is doctors checkups and preggy vitamins. Your bed to have the correct vitamins, but you can just take the lable off the bottle and say they are a normal multivitamin. Depending on where abouts you are, there are free clinics that will give you checkups without telling anyone, else bulkbilling doctors or similar.
Hope that helps a bit.
Goodluck.