I want to buy some flowers for my Chinese girlfriend do you have any recommendations and things to avoid?
Where are you? If you are in Europe, North America the protocol is identical to what you would give any other woman as they are westernised to the extent there won't be any numerical and or cultural significance.
Also you do not state if you just want to have fun or if this is serious.
Anyway TCG says unless it is serious give them anything but roses, and avoid 4s 13s 14s and any number with 4s in them. One rose is enough to imply love.
If you really want to make a gesture of it send her 10,000 roses, 10,000 pretty much means forever, probably bankrupt you though.