Here’s a couple of fairly common 8-speed gearbox types, with pics and a deliberately non-technical introductory explanation.
This post is designed to remove the fears associated with driving with an 8-speed LGV gearbox for the first time.
With good tuition and some practice, you’ll find that it will quite quickly become as familiar as any other gearbox that you already know.
GOOD LUCK. 
First up is: the “slap-over” gearbox:
I’d suggest that it’s best to think of these two lots of four gears as being arranged “side by side.”
Operation.
Neutral is much wider in a left and right sense than a normal gearbox. To save getting all those gears mixed-up, there’s a springy bit about half way across the neutral. To start off, make sure that the gearstick is in the left hand side of the gearbox, then use gears 1 - 4, which will get you to 15-20ish mph. After you’ve used 4th gear, go into neutral, then pull the gearstick to the right, overcoming the springy bit, whilst staying in neutral. Then you’ll hear a “clunk,” then you push the gearstick forwards into 5th and carry on as you’d expect.
When changing down through the gears, come down as far as 5th, then come back into neutral, then push the gearstick to the left, overcoming the springy bit, then use the left side of the gearbox beginning with 4th to continue slowing down.
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Next up is the “four over four” gearbox.
(Sorry about the hand, it was very sunny for a change!!)
I’d suggest that it’s best to think of these two lots of four gears as being arranged “four over four.” (See pics below for meaning.)
Operation.
To start off, make sure that the switch on the front of the gearstick is down:
Use gears 1 - 4, all with the switch “down,” which takes you to 15-20ish mph.
Then, whilst in 4th gear move the switch to the up position like this:
Next, move the gearstick into neutral, you’ll then hear a “clunk,” which means that 1st gear has now become 5th gear, so carry on up to 8th as you’d expect. The “clunk” means that you’re now in the “top half” of the gearbox. Now you can see why the gear numbers on the top of the gearstick are not arranged like the “slap-over” gearbox.
When changing down through the gears, come down as far as 5th, then whilst in 5th, move the switch to the down position, then move the gearstick into neutral (wait a moment for the “clunk”) and you’ll find that you’ve “switched” into the “bottom half” of the gearbox. (8th has now become 4th etc.)
In practice, with this type of gearbox, you might be in 7th and need to stop the vehicle. That’s fine, just stop and apply the handbrake. Then move the gearstick switch to the down position before going into neutral, then you’ll be in the right “range” to start off again. Experienced drivers sometimes forget to do this- myself included 
Last tip: don’t change the range and go for a gear too fast, because you might miss the gear altogether due to some range changers being slower than others.
There are other hints and tips with this type of gearbox, but I’m leaving these to your instructor to avoid any possible confusion.
They’d be difficult and lengthy to write anyway.
I hope this is helpful and calms any initial fears you might have.