I've been warned off buying a cookware set as you tend to get stuck with certain useless pieces while missing out on some important ones. If I was to build my own set, what items should I make sure to include (or avoid)? Also, any tips on best materials?
It depends entirely on your cooking style. No items is a must-have and no pieces should be avoided. If you are just starting out, I suggest that you buy whatever pots/pans that look useful to you; start with a cheap but decent brand like T-Fal.
As you cook more and more, you'll naturally know what items and what sizes benefit you the most. Then you can invest more money into getting a better version of those pieces.
By the way, a cookware set might not be all that bad. Your cooking style could happen to match all the pieces on offer. I had bought 2 sets that I'm 100% happy with everything in them.
No comment on the best material though I prefer Thermolon over Teflon for nonstick stuff.