Building up a yeast starter is always a good idea to ensure you have good healthy yeast. People have less problems with dried yeast that stores refrigerated for many years, but with liquid cultures, the yeast only has a life of approximately 6-9 months from date of manufacture before it's pretty much dead. Nowadays the yeast companies will put a date of manufacture to help you know the life better. You can also use a yeast calculator to determine exactly how many packs of yeast or size yeast starter to make for any batch at:
http://www.mrmalty.com/calc/calc.htmlThe above calculator is known to be a bit conservative but it will help you get into the right ballpark.
If/when making yeast starters, use DME and aim to make a specific gravity of about 1.040. This will grow the yeast without stressing it with very high or very low gravity.
Learn more online everywhere. Sorry I don't have any specific links. Starters are easy to make and very helpful. Not required, but nice to do, for liquid cultures anyway.
With dried yeast, you can and should safely skip a starter because it is so reliable. It is for that reason that many people including myself are trying to find and use good dried yeasts as much as possible, it's just so much easier.