Usually, I find the pattern first. Then it tells me what size needles and what size yarn and how much to buy.

Sometimes, though, I make up the pattern from a combination of stitches I like from something else. Or I buy a novelty yarn and just knit each row.

Sometimes I find I do not have enough yarn for a scarf I am making with no printed pattern, and then it seems I am running out of yarn too soon. So I rip it out and make a narrower scarf, or add a stripe of another color.

Usually, to be safe, you should have the pattern first before you buy yarn. Otherwise, you run the risk of buying too much, too little, or the wrong size of yarn.

I agree -- having the pattern picked out first is a big help! Sometimes for a scarf I just pick out a stitch (like mistake rib), and that makes picking out the yarn more fun, because it can be anything I think would look good with that stitch -- but then I have to experiment with needles to get the gauge right, and knowing just how much yarn to get is more of a challenge. So picking a pattern with guidelines will take a lot of the confusion and guess-work out.

Sometimes I find a yarn I just *have* to have, so I just make a guess about what I'll make and what I'll need for it... so far I've been ok with that but I'm bound to run into trouble sooner or later! :)

If you shop at a yarn shop, you can get some good advice -- a couple of times I've found yarns I've just had to have and the owners helped me figure out how much I might need without a specific pattern. They can also help you with yarn choices/substitutions for a pattern. Yarn shop yarn is usually more expensive but it's also lovely yarn -- worth at least doing some browsing!

There are tons of free patterns on the internet; google "free knit scarf patterns" or something and you'll have no end of links!

Have fun!

I think people are too wedded to written patterns....for me, part of the fun of easy things like scarves, is to experiment with different stitches, to see what you like...what pleases your eye. Scarves, to my mind, are great ways to 'preview' different stitches and as they are small projects, you're not 'out' much if you don't like them....(and, you can always start over). I have only used, I think, one actual scarf pattern....instead, I have a book of hundreds of different knitting stitches, which is a good thing to have in the repertoire of any knitter. I simply look through the book, pick something I like, and 'have at it'....you can also do bands of patterns instead of just one, to give you an even better idea of what different stitches will look like...your scarf becomes sort of a stitch sampler. Just be sure to use, at the sides and both ends, a couple rows of certain stitches that will make sure your scarf will lie flat...for instance, start your scarf with 3 rows of knitting (that is, no pearl rows..rows 1 through 3, knit), which will produce a garter stitch edging that won't roll....then, start each row and end each row with a few knit stitches for the same reason. There are other stitches that will do the same thing, but this is probably the easiest.
Knowing how much to buy, when traveling (and with no project in mind) is hard. If you aren't knitting sweaters or larger projects, then I wouldn't buy a great deal....when you start with sweaters, you'll get the general idea of how many you used for past projects, and this will give you an idea of how much to buy. It's a good idea, once you really get started knitting, to keep, in your purse, a list of amounts.....how much bulky, how much sport weight, etc, you used in past projects and will likely use again..but, that's down the pike for you...do keep it in mine, though. I, personally, never buy less than 3 skeins when I'm thinking of small projects.

Jenn, you sound like me when I started knitting, only I did buy some yarn, with no thought of what to do with it, and I still have it! The advice above is all good - start by finding a pattern first, then after knitting more, you'll know. It's a little like cooking. The first time I make a dish I follow the recipe, then the next time I'll make changes.

Thanks everyone. I really appreciate your great suggestions. I found links with lists of scarf patterns and I'm bookmarking them.

I just found this pattern that I really like. It's a sampler scarf using different stitches throughout. I think it would make a very handsome men's scarf.
The Road Scarf.

There's a handy laminated list of yarn requirements for various projects by Ann Budd. Many stores will also allow you to return unused skeins or balls of yarn within a reasonable time. Sometimes I buy yarn with a project in mind and sometimes I find a deal too good to pass up. Before you know it you will accumulate a stash!

Here is a link that might be useful: Handy Guide to Yarn Requirements

Thank you. That sounds like a very handy reference to have. I'm just beginning to build my knitting supplies and it appears there's no end to it!