If you're using liquid Yeast, or if you're going to re-use yeast you've washed from a previous brew, you'll need to know how to make a yeast starter. A yeast starter is the process of growing your yeast cell numbers and getting them ready to do an effective job of fermenting your batch of wort.
Here's a link to billybrew.com which does a great job of describing the process.
Here's a link to billybrew.com which does a great job of describing the process.