If the wheel is symmetrical the spokes should all have the same tension. If dished (what you've called "skew" the bicycle world calls dished), the side closer to the centre of the hub will have more tension. All the spokes on the same side of the wheel should have the same tension.
Since you have derailleur gears the wheel will almost certainly be dished (it's possible to build a frame such that this isn't necessary, but that requires other compromises, so if it's a mass produced frame with derailleur gears it's got a dished rear wheel). With hub gears it is almost certainly symmetrical, and with a singlespeed it's probably symmetrical.
In general, the spokes should be as tight as the rim allows. Jobst Brandt in "The Bicycle Wheel" is quoted in this question as saying:
With tensioned wires as spokes, the wheel can support loads only to
the point where its spokes become loose. At this point the wheel will
collapse. Therefore, for greatest strength, spokes must be as tight as
the rim permits. Structurally the rim supports spoke tension as an
arch that is compressed by the inward force of the spokes. The load
limit for most rims is far less than what the spokes could deliver if
they were tightened to their breaking point.
That book is short and cheap (but densely written) and well worth buying if you're interested in wheelbuilding.
edit: You don't need a tensiometer, you can probably do it by ear. This question discusses truing by ear and links to a useful article. Like Daniel said in the comments, focus on getting the wheel true, stress relieve it, re-true it (repeat as needed), ride it for a day or two, then check it. The closer you can get the spoke tension to even the less the wheel will go out of true when you stress relieve it/ride it, so the faster that process will go.
But if the rim or spokes are damaged you won't be able to get the tension even, and the older the wheel is the less likely it is that you'll get it even. The advantage of a tensiometer is that it speeds the process up and is more reliable (doing it by ear is very sensitive to the exact length between the rim and the first cross of the spoke, and you can change that by pushing on the spoke)
