Before jumping in, you don't need to read everything. But, you should know:
WHERE THE KEY INFO IS displayed (depth, time, NDL, deco info, gas pressure if AI).
WHAT WARNINGS LOOK LIKE (max depth, NDL, low gas)
WHAT DECO LOOKS LIKE if you go over NDL. Sometimes screen layouts change when the computer goes to deco mode. That is absolutely not the time to try to be figuring out something new.
These are the essentials to getting back healthy (or alive), so know them cold before you get in.
If your computer display is so self-explanatory that it is all obvious, great! Otherwise, it should not take more than 10 minutes with the manual to learn these essentials.
By the way, I can't say enough about DiveNav and its computer-specific training simulators. They are the best thing going for learning what your computer looks like in varioius scenarios. You know what everything looks like before you even hit the water.
Later, you can go into the manual more for any bells and whistles.