You're certainly right that it depends: on how long you want to make the scarf, how thick your yarn is, how big your needles, and how much time you have to knit. Plain garter stitch can be done with all knit stitch and it will lie flat. You'll be proficient by the time you're finished. A stockinette st scarf (knit stitch on one side and purl on the other) will curl unless you use a good sized edging of say, garter or seed stitch.
Why not find a pattern that you like online or at the yarn store and start with that?