Can I drink anything if I need to fast before collecting my sample?

Yes, in fact it is much better to make sure you are well-hydrated before collecting your sample. It will make blood-flow easier and will also mean that your results won't be affected by you being dehydrated. 

Just make sure that there are no calories (in the form of milk and sugar) in your drink. Water, black tea and coffee and fruit teas are all fine.