A common question, and one worth asking!
First qualifying question is what racquet are you using? Every racquet has a recommended string range. Lower tensions = more power and comfort. Higher tensions will = more control. Larger head sizes will often require increased tension vs. smaller head sized racquets.
Second qualifying question is what type of string will you be using? Nylon solid core synthetic gut, multifilament, and co-polyester strings all have different stiffness ratings and will come with different initial recommended tensions. In general, nylon will offer a crisp, neutral response. Multifilament's will offer a softer feel and tend to offer more energy return. Co-poly strings tend to be stiffer than both a multi and nylon and are usually strung at least 10% less in tension.
The third qualifying question is what have you strung at in the past. This is important to understand what you are used to playing with. I would also ask how long it has been since last restringing; as string tension will degrade slowly over time as the string stretches and loses it elasticity.
A typical modern day string tension range is between 50-60lbs. Most frame manufacturers will recommend around these ranges; though, they can very slightly. Many players with co-poly strings such as Luxilon and Solinco branded strings will string well into the mid to low 40's with great success. Another great option to soften the string bed of poly strings is to hybrid that string with a softer string like natural gut or a multifilament.
If you are unsure about what tension to string at, or would like advice on how to achieve a specific playing characteristic, reach out and lets have a chat. I would be happy to help you achieve your ideal string and tension combination.
All the best,