Learn the tablature staff and how it corresponds to the strings you're playing. Unlike a music staff that has four spaces and five lines, bass tabs have only four lines, which represent the four strings. The line at the bottom of the page represents the top string on your bass, which is the fattest one, E.
Study what the numbers mean. For example, 4 on the top line means to play the fourth fret on your bottom string, the thinnest one. The more familiar you are with your frets, the quicker you'll be able to understand the symbols and play the song. Learning the dots on the fretboard can help you with this. For example, if you see a 14, you should know that it is simply two frets up from the double dots.
Learn the difference between the more complex symbols, as some are related to the notes themselves, and others are merely modifiers of the notes. For instance, if you see "x" in the lines/staff, that means to play a ghost note, which is when you hit the string with your right hand, but don't actually fret it with your left; you only gently keep your left finger on it. The modifiers may look complex, but they describe a few simple actions. Most are intuitive, such as "S" meaning slap and "P" meaning pop, both of which are noted below the staff.
Look at a complex song without your bass in hand. See if you understand what all of the symbols mean. If you have trouble at first, don't fret; most tab books have a key or legend if you need to take a peek. Next, try playing the song at your own pace. Once you can do this, you will have a grasp of how to read the symbols.