I just got a jcm 900 1960 lead cab it says I could plug in into mono 4 or 16 ohms or stereo 8ohms which would be better and why? I really don't know much about ohms this is my first tube amp
Just plug your head straight into your cab?
Its not that hard?
My gear so far:

-Cort KX5 (pearl white)
-Crybaby Wah
-Danelectro Cool Cat
-Fender Twin Reverb 65' Reissue
-Hardwire TL2
the head will dictate what cabinet settings to use. if the head is variable output impedance then most people option to go with the highest impedance possible. the reasons for this are evidently tonal from what i have heard, i believe people claim the signal has higher potential for a less noise when running at a higher impedance.

either way, impedance from head must match the impedance of the cabinet for proper operation. so if the head is set for 16 ohms, then you want a cabinet that is 16 ohms.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
Right, keep the cab on mono, and plug into either the 4 or 16 ohm jack, depending on what your amp is set to.

In this case, I would go 16, not necessarily because you want to use the 16 OT tap, but because it's using series/parallel wiring, as opposed to parallel, which most people prefer.

The wiring makes a bigger difference in sound than which tap, but Gumbilicious is correct, some people debate the tonal differences.

As has been stated, just be sure to match them.
The stereo 8 ohm jacks are for stereo applications only, which can't be done using just a JCM.
thanks rutch that cleared alot up im gonna try both 4 and 16 on mono and see which one i prefer