If you have never tried knitting with beads, I hope this will encourage you to do so! It isn't as hard as you may think.
Beads come in many sizes, shapes, colors, and are made from different types of materials. You can find them in craft stores such as Joannes and Michaels; your local yarn store may even carry beads. There are also plenty of places online to find beads.
One thing to consider when choosing your beads, is whether or not they will fit onto the yarn you want to use. If you are in love with a particular bead, but your yarn is too thick to fit through the hole, you can use a thinner yarn to put the beads on, and knit with both strands. Choose a complimentary color, or a contrasting color - the choice is yours. If you only want a few beads here and there, sewing them on afterward is also an option. Something else you may want to consider is how much weight the beads will add to your finished item. Using lots of big, glass beads for example may make your scarf uncomfortable to wear.
There are probably as many ways to add beads to your knitting as there are knitters. The most common is stringing the beads onto your yarn before you begin knitting, or adding the beads one at a time as you go along with a crochet hook. The second method is the one I prefer. My problems with stringing the beads on beforehand are: A) If the yarn is a fine, thin yarn the weight of the beads may break the yarn B) The moving of beads up and down the yarn can cause a bit of fuzziness or pills on the yarn. If you are using a fine, delicate yarn and prefer to pre-string your beads, just do a few at a time. When you run out of beads, break the yarn, add more beads, and continue. This will also leave you with extra ends to weave in, which can be a pain.
As I said, I prefer adding the beads one at a time with a crochet hook. This method is the fastest and easiest for me, and the beads appear equally well on both sides of the fabric. There is no worry that a bead may be stubborn and want to show itself on the wrong side of your fabric! To add beads this way, you will need a crochet hook that is small enough to fit through the hole in your bead. I use a US Size 11 hook, which is teeny tiny. Since it is so small, it tends to snag on thicker yarns sometimes as you pull the yarn through the bead. If that happens, all you have to do is pull the rest of the plies through the bead, and it will be fine once the stitch is back on the needle and you have knit it.
You can add beads to any knitting you want, any type of stitch. Don't be afraid to add beads to jazz up a pattern from a book or an individual pattern that you have purchased. There are no rules.
When adding beads with a crochet hook, simply knit to the stitch where you want to place a bead. Then, slip your bead onto the crochet hook. Next, pull the stitch off your left needle, and through the bead. Once the bead is on the stitch, place it back on the left needle and continue knitting. Continue this way, adding beads wherever you like!
I just add my beads whenever I feel like a bead should be added. If you like to plan ahead, get some graph paper and plan out each point where you would like to add a bead, and use that as a guide as you knit. If you are feeling especially creative, you could even chart out a motif for your beads.
Below are a few links where you can purchase beads online, and a brief video of my preferred method of adding beads. Enjoy!
Fire Mountain Gems and Beads