If you're talking about the type of wood . . .
I usually use pine because it's cheaper. It's also much easier to work with than oak, which is very hard. Try to get wood that has few knots & is flat. Sometimes the wood is VERY bowed - not good for shelves. Look down the length of the board as well as across the width. Also, try to get wood that isn't terribly "green" - meaning full of sap. The stronger the smell, the more sap usually. I've already gotten some that almost felt "wet". It's best to leave it sit for a bit before starting your project if you aren't sure it's well seasoned (dry.) If you make your shelves while the wood is still wet, it may bow even though you've painted or stained & varnished.
I hope this helps.
Deb