HOW TO CHOOSE THE RIGHT TUBE
Find the numbers printed on the sidewall of your tyre. There should be two numbers separated by a dash. This is ETRTO sizing - the first number relates to the width of the tyre, and the second to its diameter. There should also be two numbers in brackets. This is imperial sizing - the first number relates to the nominal diameter of the rim, and the second to the width of the tyre.
Find the tube with the corresponding numbers printed on the box. Try to find an exact match if possible.
Choose the right valve! There are two types: Schraeder (AV) sometimes called "normal" or car valve, and Presta (SV) aka French valve/the long skinny one. Check which one is currently on your bike, and choose a new tube with the same valve.