I've had this question a couple of times since selling my designs on eBay so I thought I'd best write a guide. I hope it makes some sense! I'm always happy to try to answer any emailed questions about knitting. You don't have to buy anything to get an answer!
Types of stitch markers and other ideas
Stitch markers come in many forms, they either slip over the needle or can be attatched directly to the knitted or crochet stitch. I prefer the beaded variety as they are pretty but you can use many other household items, dental floss is a good cheep alternative (it can also be used to hold stitches, just thread a strand through and tie in a knot instead of using a standard stitch holder and the cutting strip will cut most yarns making dental floss a handy thing to have in your knitting bag!), I've also used ordinary finger rings and once my own earring (one of the BCR types you can find in piercing shops) when I needed one on the move!
A little bit about how and why to use them
For a good few years I ignored the bit in patterns that says 'cast off two stitches, add marker, cast off two knit to end' as I wasn't really sure what they were on about. Now I use them all the time to aid in matching cast off edges when sewing up and keeping track of row ends in socks. They are handy for keeping your place in patterns and great for making sure your sleves line up and your V neck ends up in the middle! You will get a much more professional look to your made up item.
Ring stitch markers are slipped onto the needle to mark the end of a 'row' in circular knitting or to highlight where a particular bit of pattern needs to be knitted in along a row. Simply knit up to the marker, slip it over to the right hand needle, and then carry on.
Clip stitch markers are best for times when the stitch itself needs to be marked. You can clip them directly into the stitch loop and then unclip them when you are done.
Th last bit.
Be Warned!!!!!!!! Don't have a stitch marker disaster!!!!!!! I avoid closed ring stitch markers as it is possible to make a slight mistake and knit your marker into your knitting stitch, your only two options then are to leave it there and pretend it's an embellishment, or unravel your knitting to release it. With split ring markers there is a third option, open up the ring with a small pair of pliars and slip the marker out. You can then close it again and carry on using the marker.
Most importantly, experiment! Peoples knitting styles are very individual so have a bit of a 'play' and see what works for you.