Making walls to divide a room

Don't know whether this is the proper place to post this so apologies if it isn't. I have just rented a fairly large 3 story house and need to know how to make dividing walls as the rooms are quite large. Can I just buy them or do I create (the box) and then stretch to suit. Also how do I then make a doorway? Any help would be appreciated and especially a walkthrough or pointer to the relevant video help. Thanks
PS I am still confused about the diff between prims and textures.

Primes are objects (the wall) textures are pictures you apply to the object (wall) to make it look painted or wall papered or whatever.

You are going to have trouble matching the new walls to the original walls as the creator has the textures to match the new walls and you don't. So your walls are not going to look like the originals. If the house has mod rights you can change all the walls to your own textures.

Look up "Ivory Tower Library of Primes" in search (in SL not here) and go through the tutorials and stuff.

The sort of modification you're looking at is VERY easy, so you don't need any tricky building skills. To make a wall, just take a cube prim (not "prime" -- the word "prim" is a short way of talking about "primitive" building blocks), flatten it, and stretch it to fit the space. Bingo. To make a doorway, you'll need to make your wall out of more than one prim, the way you made doors when you were playing with blocks as a kid. If you really want to put a door in the doorway, that will take a little more skill, but nothing terribly difficult. You'll find ready-made free doors in many places around SL that you can just pop into place in your structure. Try Yadni's Junkyard for starters.

Textures are applied to the surfaces of prims. Rather than try to match whatever texture the builder used for other walls in the house -- probably impossible to do -- the easiest thing to do is leave them blank (not the default wood texture) and apply a pleasing color. That's a simple job that takes a couple of seconds with SL's Edit tool.

This is a good excuse for you to learn basic building skills. As Lyla suggests, the Ivory Tower of Prims is a nice place to find straightforward tutorials. You might also consider taking an in-world class. Use your Search function (then "Events >> educational" to look for them. There are usually several a day to pick from.

Good luck, and have fun with it.

Thanks Lyla and Rolig.
I will follow your advice and am looking forward to learning new skills.