[FONT="Times New Roman"][SIZE="3"][COLOR="#201090"]Matt,
The only answer for all of your questions is that "it depends." Both the yarn (weight or thickness) and the needle size affect the size of your stitches and therefore the size of your finished object (FO).
To start out, you need yarn, needles, and a pattern for your scarf. First look around this forum, the web, and in magazines for a pattern that appeals to you and then it will give guidance on the yarn weight and needle size. I can't help much on the yarn weight, all I know is thread is small (like from sewing thread up to average string size), "sport" is a thinner yarn and "worsted" is a thicker yarn. Yarn weight often pairs up with needle size (as in "size <whatever> is likely the best/smallest needle you would want to used with this weight yarn"). Choosing a smaller needle gives a smaller/tighter stitch and a stiffer fabric, where as a larger needle gives a larger more open stitch that is softer and your FO will be less stiff.
There is also the yarn material (silk, cotton, synthetic, wool, others and blends of these), the color, and the sheen of the yarn to consider. Take a sister, ask a sales associate, or a random female in the store if they can tell you what "so and so" meant when they said "worst-ed" yarn. Then pick up an ugly skein of yarn and ask if this is the worst and why would "so and so" want this anyway? Note to self to try this to see the reactions. :roflhard:
Oh yeah, a backpack is a good place to hide your work. If you ever feel the need to hide your work when your not working on it.
Finally, you start by casting on, and you need to practice. Use the excellent videos that Amy has provided on KH and try the stitches along with the video. Expect to make mistakes and have to pull out (also known as ripping out or FROGG as in "rip-it" "rip-it") back to the beginning and starting again. I tend to have tight stitches, so I usually use one size larger needle (or hook as I crochet more than knit) than the pattern indicates.
Were you looking for any of those answers? I hope so and good luck.
P.S. As a Crochet person and new to knitting, I wonder. Why, oh why, don't they sell needles in package sets with several sizes. What is a good selection of needle sizes and variations (double point, circular, etc.) and why/when do you need these other variations?[/COLOR][/SIZE][/FONT]