When I was a newbie knitter, I had a theory that I wouldn't be a real knitter until I'd made a sweater. I decided to start with the simplest construction I could. The easiest way to create a sweater in one piece, requiring no seaming, is to make a sweater with raglan sleeves. Raglan sleeves are sleeves that start at the collar and slope down to form a diagonal effect. For example, think of a jersey style t-shirt.
Raglan sleeves start at the very top of the sweater, with the work of separating them out starting on the very first row, or close to it. Very early on in your project, you will place 4 stitch markers at designated spots on your needles. Then for several inches, you will increase one stitch on each side of those markers, 8 total increases per right side row. Over time, this will create the appearance of 4 diagonal lines down your project. The area between the first and second marker will be one shoulder piece and the area between the third and fourth marker the second one.
At some point, the sleeves will need to be set aside as you continue working the body of the sweater. At that point, you will slip the stitches that will become the sleeves onto stitch holders and continue working the body of the sweater. This point corresponds to the bottom of the armhole. Then when you're done working the body, you just come back and slip those stitches you set aside back onto needles and work your sleeves. No seams, no sewing, no picking up a bound-off edge.
With raglan sleeves, one of the trickiest aspects is not leaving a hole in the fabric at the bottom point of the armhole. As you separate off the sleeve stitches, you're suddenly trying to knit across a gap where perhaps 40 or more stitches had previously been. The stitch bridging that gap can become very stretched out. While you want to do your best to knit those two stitches as close together as possible, you can address that problem on the next row. As you come back to that spot, that vertical bar between the stitch on either side of the armhole will be longer than usual. You can wrap that bar around your needle a time or two and knit or purl it with your next stitch to help eliminate that gap. Even if there is still a gap, never fear. There will eventually be a yarn end to weave in at that spot. While weaving in that end, you can further close up the gap.
The signature aspect of a knitted raglan sleeve is the diagonal line slope down from the collar. Because that effect is created with increases, you have two options. You can either make increases that leave a hole or you can twist your increases to make sure there is no hole. It is an aesthetic choice that will depend on things like your overall pattern and your yarn choice. If you're making a sweater with some lace detail, for example, the hole effect down the sleeve lines can fit right in. If you're making a bulky sweater, though, you might want to avoid aiding an airy detail.
To me, the best thing about raglan sleeves is how effortlessly they take shape in your garment. You'll hardly even realize you're making sleeves. What's your favorite thing about raglan sleeves?