There are loads of good guides online for making starters. The basic principle is you make a very simple wort of close to 1.037 SG (1L water to 100g DME) with a light malt extract and pitch your yeast into this, let it ferment out then either pitch this into your wort (your final beer wort) or you can chill the starter and pour the beer off the yeast and just pitch the yeast slurry.
You first use an online calculator to work out what size of starter you need, depending on your beer. Lets say you need a 1L starter, then you will boil a bit over 1L water (some will boil off) and 100g DME for 15 mins. Put the pan in cold water until this "wort" is down to pitching temperature then pour into your flask (erlenmeyer type is ideal) and add the yeast. after about 24 - 48 hours the yeast will have fermented out and your starter is ready. This can either be pitched directly as is or you can cool it down more and pour off most of the fermented wort from the top and swirl the yeast back into the liquid and pour in.
Basically what you are doing here is multiplying the yeast cells to get the desired number for your brew.
Obviously everything coming into contact needs properly cleaned and sanitised before use. The process is very simple but I would advise reading some of the very useful guides out there. As I said before I also suggest overbuilding the starter and saving some for future beers.