Fiery Vixen is absolutely right-- with scarves you can use pretty much whatever you like! The one other thing I would add is that on the label of the yarn you choose, it will give a tension example-- X amount of stitches and rows over a 10cm square. You aren't going to need to worry about the rows at all-- because you're just going to stop knitting when it's the length you want! But you'll get at least a rough estimate of how many stitches you'll need per 10cm. So if they say 15 sts, and you want a scarf 20cm wide, then cast on 30, as FV said. Or you can adjust for whatever you want. One thing I would say is that be prepared to rip it out after a few cm, because when you're actually knitting, it might come out thinner or wider than you had imagined
. But then you can go back and adjust your number of stitches to make it come out exactly as you imagined.
She's also absolutely right about stitch pattern-- if you knit one row and purl the next (stocking stitch) your scarf will curl at the ends and edges and probably make you miserable-- we've all been there
. Straight knitting every row (garter stitch) will stop that, and is the easiest thing for a beginner.
The one thing I do differently is to leave about 20 - 30 cm of yarn at the ends of my yarn changes-- that way you have a bit more to work with when you weave in the ends, and it's easier.
Just make sure you get enough yarn!-- inevitably, if you run out, you'll go back to get more and they'll be out or at least out of that dye lot! It just seems to be karma!
To be very sure, you want probably want maybe 150 metres of each colour.
Good luck with it!