oh boy where to begin...
Plasticard you can get from places like Bits&Kits. You're in UK right? You might have a Modelzone shop near you, they might have some. Otherwise recycle packaging is always a good option (although it's not the same as true plasicard).
Greenstuff you can get online. Modelzone will have Milliput. But if you're a beginner converter and perhaps sculptor, maybe go for milliput as it's good for gap filling, it's cheaper and larger quanitity than GS.
Things to convert:
Try to begin with just altering your existing models. Use old ones as experiments. And start getting into pinning every joint on new models as you build them. That'll get you into practice for posing them in dramatic postions later.
Common areas to convert are things like; Heads, Weapons, Poses, then adding stuff. More advanced is when you start to remove stuff, hacking and merging things. This is when the milliput comes in handy. More advance than that starts to involve fabricating around parts, trims, details, some thought upon layout/design.
There's loads more that could be added, but I fear I may waffle into 'become one with the minature sky converter'. Woops already done it, sorry.
Bases are a good place to start - Try to add to the overall model without actually adding to the model itself is tricky. But it'll get you into the mindset of what 'suits' the model. And sometimes the environment would have an effect on the model, so it could give ideas to how to convert the model. EG; Acid swamp base may = Trooper is corroded/ melted/ burnt. Jungle base may = camo on trooper with added leaf/net cloak.
Other tips; Dry fit everything before gluing.
Take a look around, there's tons of conversions on Heresy. Perhaps look at the previous conversion contests as they'll be full of converts.
Have a 'Bits' box and destructo tools, I mean Dremel, saw, vice, drill vice, knife, clippers, sandpaper.
Do designs/drawings if'll help before hacking away at things.
Keeping things simple is never a bad idea. A simple gun swap can be considered a conversion and can make all the difference. Doing too much sometimes just confuses people as to what they are seeing.
Don't expect you projetct to be finished in one weekend. Some conversions can take over a year, and that's not including painting afterwards.
And don't forget to take lots of pics and post them up in a thread!
Good luck sky converter!