When I posted about my 'Roses are Green' tunic, I was asked about any tips I could share on sewing with knits. I was going to include this with my next knit project post (coming soon!), but I realized I've got more to say on the subject than I initially thought, so this will be a separate post. :)
First of all, it's a good idea to familiarize yourself with different types of knit fabrics. Oliver + S had a good blog post about this, "some knit fabric basics." I would recommend visiting your local fabric store and checking out some knits in person. Look at the fiber content; examine the stretchiness, drapiness, and feel of the different fabrics. Take a look at some of the knit garments in your own closet and see what kind of fibers they are made from, examine the seam finishes, etc.
To be honest, I have never made a knit garment from a commercial pattern. The first time I ever sewed with a knit, I took apart an old knit shirt and used it as a pattern. I think this is a good way to start out because you already know that it will fit you well -- just choose a fabric that is similar in weight and stretch to the shirt you are using as a pattern. If you have a "master pattern" like this that fits you well, you can really leap off from there and design anything you want!
As far as the actual sewing goes, I've learned by trial and error. Be sure to sew with a ball point needle in your machine. Some machines have a "stretch" stitch and I have heard recommendations that you sew all seams with a zig-zag stitch -- personally, I have sewn most of my projects with a straight stitch and I haven't found the stretch of the knit to be compromised. Experiment on some scraps if you are unsure what kind of stitch to use. I do have a serger so I finish my seams with that. If you don't have one, you can zig-zag the seams together and then trim close to the stitching, or even just leave them unfinished if you are feeling lazy. :) Knits don't unravel the way woven fabrics do.
Sometimes it can be tricky to feed the knit fabric through the sewing machine without stretching it. I think some machines have the option of lessening the pressure of the presser foot, which may help, but my machine doesn't (or at least I haven't figured out how to change it!). If I find that the fabric is becoming distorted as I feed it through, I will stop with the needle lowered and raise the presser foot to release the fabric. You may have to do this often.
A trick I learned relatively recently is to reinforce shoulder seams (and any other seams that are likely to receive a great deal of tension) with clear elastic. You can read more about that here.
Another feature you'll often see on store-bought knit garments is a double row of topstitching. I used to think I had to just sew around a hem twice to get this look, but I learned that it actually comes from using a twin needle. This has the added benefit of creating a nice finish on the wrong side of the fabric. You can just trim away any excess fabric near the line of stitching.
(I began to type out instructions on how I typically bind the necklines of my knit garments, but then I realized that without pictures, you're likely to just feel very muddled. :-P So if you are interested, let me know and I can do a photo tutorial.)
Whew! That's all I can think of for now. If you have any additional tips of your own, please share with us in the comments. And if I didn't address a question you have, feel free to ask! I have had my share of flops, so I know it can be discouraging when things don't turn out the way you envision. But honestly, knits are forgiving! I encourage you to give them a try! :)