Welcome to the forum Ben
Lets see if we can help ya.....
A lot of new riders think that there is a "reserve tank" on a motorcycle which is actually a separate tank from the main tank, but it isnt.
This picture shows you how the tank is actually set up:
The yellow arrow points to the petcock intakes (or straws).
The blue area is the "fuel reserve."
The red area is unusable fuel. T
he white area above the blue, inside the tank, is the "normal" fuel.
There is no separate reserve tank.
Most bikes have only one fuel tank (its that big metal thingy you straddle when you get on yer bike and sometimes slap with yer knackers )
Most bikes that have a "reserve" tank will have 3 (or possibly 4) settings - On, Off, and Reserve
The OFF position shuts off fuel from the tank to the carburetors, and is how you want to leave the bike if it's going to sit for a long time.
The ONposition is where it should be set for normal riding. This setting allows the fuel from roughly the top 80% of the tank to flow to the engine. The way it does this is by putting a tube a few inches up into the tank, so it can't reach the fuel on the very bottom.
The RESERVE position is just like finding a longer straw. It allows you to get that last 20% of the fuel in the tank. It's not a separate tank, just a separate straw.
If you are out riding and your bike starts to run out of fuel you can switch it to Reserve and it should be enough to get you to the next petrol station to fuel up.
Make sure you turn it back to the ON postion before you set off though or else you could find you run out of fuel altogether !
Hope this helps.