The tuba is really just a series of tubes that you can use to play different pitches. This page shows you all the pitches you are able to play with any given valve combination. You'll notice there's a pattern in the notes; the intervals between them are all the same. This is called the overtone series. All brass instruments work based on that pattern.
One valve combination that isn't featured on the page above is the fourth valve. The fourth valve was added to help correct for intonation problems that arise naturally in the overtone series.
What kind of tuba do I have?
Just like cars, or shoes, or anything else you can buy, there are lots of different brands and models of tubas. (Some even come in different keys- but if you are not playing a Bb tuba, you probably know all this already! Bb is most common, so we'll focus on that for now.) The main difference we are concerned with here is whether or not you have a fourth valve. Check out the examples below (courtesy of woodwindbrasswind.com) and see which one looks most like your instrument (regardless of color).
Four-valve Bb tuba (with rotary valves)
Three-valve Bb tuba (with piston valves- this one happens to be a 3/4 size tuba, usually for beginners)
Four-valve Bb tuba (with piston valves)
When do I use the fourth valve?
Use the fourth valve instead of 1-3. Instead of being very sharp, the resulting pitch will be much more in tune. Using 2-4 together instead of 1-2-3 will help solve intonation problems with that combination too; the fourth valve also helps extend the low register of your instrument. If your tuba doesn't have four valves, there are some intonation problems you'll have to solve yourself; some players use embouchure or air to adjust those notes, and some players use tuning slides if they can move quickly. Some instruments even have that option built in! Talk to your band director to figure out an option that will work for you. For more intonation fixes, see the tuba intonation page.
Want a standard fingering chart?
This chart is organized chromatically, includes fingerings for Bb and C tuba (written as BBb and CC), and includes fourth valve options.