You can go here to this site hun, www.yarndex.com
and then look for the yarn you are wanting to sub. There it will tell you the weight of the yarn and the gauge or tension, which are very important when you are subbin yarns. Then you can find other similar yarns with the same weight as the one you want to replace.
There are generally 9 weights of yarn, carry-along (very, very light weight, also a lot of novelty yarns fall into this category), lace weight, fingering or sock weight, sport weight, dk or doubleknitting weight, worsted weight (most popular), chunky, and then bulky.
There are some that fall in between or are novelty type yarns. But most yarns can be put into those groups I just mentioned.
The yarndex page will help you find suitable yarns for your projects. You'll want to look for stitches per inch to match up on the same size needles.
And whether or not you want texture or not. (slubs, boucle, thick n thin, fuzzy, etc....)
Hope that helps ya hun.