I agree with the suggestion of acquiring a terrain book. the
GW "How To Make Terrain" book will give you most of what you need to make good looking serviciable terrain without overwhelming you with hyper-detailed instructions that are better suited for model railroads.
I can't speak to youtube, but here's some very good sites for terrain building.
Necromundicon is a great source for learning how to make terrain from bits, toys, junk, etc. www.ironhands.com
As well as great pics and how-to's it's got construction tips, supplies lists, etc.
http://ironhands.com/
TerraGenesis.
An entire forum and archive dedicated to terrain building.
http://www.terragenesis.co.uk/
Matakishi's Tea House. Lots of neat cheap terrain projects and really good photography.
http://matakishi.com/
Lastly, if I may toot my own horn, I've made quite alot of affordable terrain with good results. Plug "terrain" into a search of our club's blog to see articles on buildings, hills, caverns, ruins, wargames table, etc.
http://chicagoskirmish.blogspot.com/
For some basic terrain supplies, I recommend first deciding what kind of terrain you want to build and then collecting what you need. Otherwise, you're likely to end up like my porch which is filled with boxes of stuff for terrain building.
If you want to build hills, all you need is foam, texture paint some small gravel and/or flock and maybe a stiff base.
If you want to build exotic urban ruins, there's a whole different collection of supplies necessary.