How attentive am I?
Before you start seeds you should know that they need a lot of attention - seeds are basically divas. If they dry out, they are usually toast. Too much water and they are toast too. Until they are a pretty good size, seedlings are incredibly fragile and you don't have much latitude with their care. I have killed more than I care to admit, so if you are going to be away or don't want the hassle of babying a bunch of needy seedlings, consider spending the money to buy full blown, robust plants.
How ambitious am I?
While you may be tempted by seed catalogs, don't order more than you'll need. And make sure not to start more seeds than you'll be able to care for. Remember that even two tomato plants in containers can give you lots of tomatoes. Also, if you're a beginner start with easy seeds like nasturtiums, tomatoes, zinnias, cosmos, morning glories, cukes or squash. Lots of herbs are also easy to start. Try basil or coriander. Dwarf sunflowers are also fun.
Do I have enough natural light or will I need artificial lights?
Seedlings that don't get enough light, become spindly, wimpy plants that won't thrive. Chances are high that your seeds will not get enough natural light - 6-8 hours a day - in a window. If you don't have enough natural sun, which usually means a south facing window, you will have to provide artificial light. You can use grow lights, which are available at hardware stores, catalogs or nurseries. For a more economical solution, you can use fluorescent shop lights hung from a movable chain. This will allow you to raise the lights as the plants grow. If you're using this set up, put in one warm bulb and one cool bulb. Also, keep the lights very close to your seedlings - about 1 or 2 inches above them.
Putting artificial lights on timers is also a good idea and set them so that the seedlings will get 12-16 hours a day of light.
When is the last frost?
Can I protect my seedlings from frost?
Can I count backwards?
Now you'll want to figure out when to start your seeds. First get out a calendar. Take your last frost date (see above), or when you want to put your plants out and start counting backwards, usually around four to six weeks, depending on your seeds.
For example, if your seed package says that you should start your seeds four to six weeks before the last frost, and you want to put your plants out at the beginning of May, you would count six weeks back, which would start your plants in March.
There is a great interactive "grow guide" from the "Weekend Gardener," that can help you choose dates. This is particularly helpful if counting backwards is not your strong suit (I'm raising both my hands here).