Most patterns will either list a weight of yarn to use or will list the yarn used on the original. You should be able to google an actual yarn name and find out its weight. Then I look at web sites to find another yarn that's the same weight and a similar fiber content. Make sure you get the same amount of yardage as the yarn used on the original, though.
*Dream Swatch in Madil Eden Bamboo
On the needles:
*Isabella from Spring Knitty
*Striped hoodie from Nursery Knits
*Tomato from Knitting Daily
I'm hunterjenn on Ravelry. Come find me!