But this leads into another question: "What kinds of strings are best?" --and this has many different answers. For banjos and especially for guitars and mandolins, there are a bewildering number of choices in strings: light gauge, medium gauge, or heavy gauge; nickel-wound; bright bronze; phospher bronze; "bluegrass" alloys; and the modern high-priced, long-lasting string sets. Which do you need?
If you like your old set of strings, I'd recommend sticking with the same kind when you change them. But if you'd like to try something new, there are a few general guidelines you can go by when choosing strings. Usually, medium-gauge strings provide more volume but are not quite as easy to play, but there are exceptions to that. And very old (pre-war) Gibson mandolins or Martin guitars may really need light-gauge strings, to avoid putting too much string-tension on a fragile instrument. In any case, on banjos, light-gauge strings often sound and play best.
On guitars and mandolins, phosphor-bronze strings may provide the most volume and bassy tone, but also may have the shortest life before they go dead. Nickel-wound strings may give less bass, but may last the longest. "Bright" bronze strings, my personal favorites, may be somewhere in the middle. The new "long-life" string brands seem related to bright bronze, and they do last a long time, but they sometimes seem stiff and difficult to play. And you'll find instruments, and different string brands, and individual string-sets, which will surprise you on all these counts!
If you have the time and energy, try different kinds and brands of strings until you find the ones you like best. If you don't want to be changing strings lots of times to find the right ones, ask around, especially among folks who have been playing a while, to see what kind of strings you might like. (Be aware that usually the answer will be the strings THEY like, not the ones YOU might like, but you can filter the answers and figure out what to try.) Good luck!