It depends. Do you like the strings that are on it? Do they need to be replaced? Do you know the type of strings that are on it? Basically, strings are sold in certain gauges, meaning how thick they are. There are other factors which go into the strings, like how they are wound, or what exactly they are made of, but the gauge is the main thing. To find out what gauge string is on the guitar, either bring the guitar or one of the strings (tell them which one) to a music shop and ask for a set of the same gauge. Alternatively, you can use a measuring device like a calipers or micrometer screw to determine the gauge of the strings yourself (it is the diameter of the strings which is quoted, and it is in inches. For example, a 4 string set which is quoted as 45 - 105 will have the thinnest string at .045 inches and the thickest as .105 inches. If you don't like the set which is on it, then try to think would you like heavier or lighter strings. If you can find out which set is currently on it, then you can try the next set up or down. You might need to have your guitar adjusted to account for the change in tension. Ask about this in the shop. Finally, if you think it is just the sound which has gone from the strings, then try wiping the strings with rubbing alcohol (be careful to not get any of this on the fingerboard) and/or removing them altogether and boiling them in a pan of water for a few minutes. This is said to remove the dirt which accumulates between the windings and brighten back up the sound. If you remove them altogether, release the tension fairly slowly (take your time) from the bass, put them back on as soon as you can, and build up the tension again fairly slowly. I have seen players snip off old strings with a pliers which makes me cringe. Best of luck.

I would agree with what 'TheStuff' says, adding only two things. A lot of the tone of a set of strings depends on what they are made of. You have three basic options (Nickel, Steel, and Stainless Steel) in two basic formats (flatwound, roundwound). Nickel usually produces the warmest sound of the three metals, while stainless produces the brightest, with normal steel somewhere in between. One advantage of stainless steel over the other materiels is that it does not rust. The other major determining factor is how a string is wound. All strings are made of a core, with nickel, steel, or stainless steel wrapped around the core. Flatwound strings use a square wrap, while roundwounds use a round wrap. Flatwound strings are usually recommended for frettless basses, as they do not damage the fretboard so much. They also produce a VERY warm, thick, almost muffled sound. Roundwound strings, on the other hand, produce a bright, almost sharp tone, especially when brand new. They can damage your fretboard if your frets are too short though. I personally prefer stainless roundwounds for my fretted bass, and nickel flatwounds on my frettless bass. God Bless.

Fodera Victor Wooten Signature Series. They're pretty expensive, but they're the best strings in the world.

I'm partial to Ernie Ball strings.