Idiots guide to putting up a garden fence needed!

[MummyPigLovesAppleSauce]

We have a 2ft retaining wall in our garden and I want to put a fence across it to stop DC falling off. I have a week off next week to get it done but I have no idea where to start!
That bit of the garden is 18ft across and I've decided I want trellis rather than fence so I can see through it and grow plants up it.
Right, clever mumsnetters, what do I need? How do I calculate how much fence I need? What about fence posts? Most trellis I've seen are 6ft long so I'd need to cut it down... how do I do that without wrecking it? Do I need to set them in concrete? How do I do that?!
Complete diy novice here but I'm determined to do it and do a good job!

YouTube is brilliant for how to videos so I would start there. There is bound to be project similar to yours and you can look up 'how to cut trellis to size', etc.

You tube didn't even occur to me! Thank you!

YouTube is great for DIY!

As for fences, I'm no help! DH said he needs to put some more posts in ours so I said we'd borrow a post basher from the farm... turns out domestic fencing isn't erected the same way as horse fencing!

(unless you think a strand of electric fencing will keep the children off. I can advise on that!)