RAM slots are usually color coded and numbered. Your motherboard manual will tell you which slots are which, and should tell you which slots to use for best performance.
This motherboard is typical, in that the slots are color coded and arranged in pairs.
Usually, what would be done is install RAM in the two yellow slots first, for proper dual channel operation. If an additional stick or sticks is installed, it would go in the orange slots. Most of the motherboards I've used were blue/black, but it's otherwise the same thing.