1. Find a pattern or design your own (see Elizabeth Zimmerman's percentage system in Knitting Without Tears). Consider style ease of construction (I like seamless!), and availability of materials.
2. Choose an appropriate yarn. There are several factors to consider in addition to cost:
Fiber--wool is great. Wool blended with nylon is appropriate for garments that will have to take a lot of hard wear. Fine wool, cotton, linen, or blends make good summer clothes but may be a bit harder for a beginner to handle.
Texture--garments worn next to the skin should be made of soft yarn while crisper wool would be good for an outer garment such as a sweater jacket and would probably wear better. Slub or boucle yarns add interesting texture to plain sweaters but might hide a lace or pattern stitch. Fisherman sweaters and lace should be made of smooth wools to accentuate the pattern.
Weight of yarn--Bulky yarns are quite warm and heavy and will knit up quickly. Finer yarns take less ounces of wool but require many more stitches. Many outdoors people prefer to layer clothes, wearing several lighter sweaters, so that they can be comfortable over a range of temperatures and degrees of activity.
Color--Choose a color you like and which fits in with your wardrobe, a well made sweater lasts for years. Use color patterns for coordinating with several outfits. For example, a few violet patterns in the yoke of a sweater will be inconspicuous until you wear it with a violet skirt or slacks. Neutral colors are the most versatile. Children seem to like the bright primary colors. Heathers and tweeds are easy to keep looking clean. Often a pattern will suggest a specific yarn.
Ease of care--some wools can be washed and dried 3oy machine while at the other extreme hand washing in cold water is necessary for others. Check to see if the wool is moth proofed. If not it will have to be stored in a cedar chest or with mothballs over the summer.
[I no longer use moth balls because they are too toxic. Instead, I make sure to wash clothing well at the end of the season (which can be a lot longer than you might think in Wisconsin), before storing. ]
3. Check the size, making necessary alterations. Measure a sweater that fits well. Shorten or lengthen sleeves and hems by working fewer or more rows. Chest size is determined by how many stitches you cast on. It is possible even to "try on" the garment before its done by carefully pulling it on with the needles in place or by putting the stitches on a piece of yarn.
4. Make a swatch to check your gauge. If your gauge differs from that called for on the pattern and the yarn label, use smaller or larger needles.
5. Plan for some personal touches such as textural patterns, color patterns, embroidery, monograms.
6. Now you are ready to start knitting!
7. Finishing consists of sewing or weaving the pieces together, binding in the loose ends, adding finishing touches such as buttons, embroidered patterns, and doing any last minute alterations. If a neckline seems too slack, thread a double strand of elastic thread through the backs of the edge stitches to snug it up. Often at this point the sweater doesn't just look quite right, it needs to be blocked. If you plan to wash the sweater, clean according to directions then shape the wet sweater on a flat surface (a towel on a table is good) and allow to dry. A steam iron can also be used by covering the sweater with a press cloth (pillow slips are good) and gently patting with the iron set at "wool" and steam. Allow it to lay flat until dry. Be careful with ribbing. Don't steam it as this will stretch it out. When washing the sweater, bunch the ribbing together when you lay it out to dry to keep it elastic.
8. Care of woolen sweaters . I prefer to hand wash in lukewarm water with mild soap or detergent. This can be fairly easy if you have a large basin or bucket to soak the garments (sorted by color, of course) and a washing machine which can be set just to spin. Allow the soaking to soften up the dirt and then just rub the heavily soiled spots. Too much agitation and rubbing will make the wool into felt. Next lift the garment from underneath so that the weight of the water doesn't stretch it out and place it in the spinner. Spin out the water and then rinse and spin again. Spinning out the water makes drying a lot faster. Immediately lay out the garment flat to dry so there are no wrinkles being set in. If you don't have a spinner, squeeze out as much water as possible by hand, then roll the sweater in one or more towels and press out the water. Store the sweater folded rather than hanging it on a hanger or hook.
Often allowing the sweater to air out by laying it flat or draping it over the back of a chair will freshen it up so that it doesn't have to be washed as often and gets its shape back.
9. Wear and enjoy!