How do I use different yarn than what the pattern calls for?

Ok, stupid question but I already found out that I get answers in this joint!!

I want to make a tank top that calls for a cotton blend yarn out of a ribbon yarn…is there a formula for converting different yarns to fit the pattern guage or do I have to hurt my brain trying to figure it out?
Help!

Thanks so much!!

Dain

p.s. I finally finished my first sweater and couldn’t have done it without the help of the good folks here! (will post pic. soon)