Depending on what you planted and when, your seedlings may need to be planted in a larger pot before you transplant them outside in your garden. When your seedlings first sprout up, you will see two leaves emerge - these are called cotyledons. The cotyledons are actually a part of the seed and act as a food source for the sprouting seedling. As your seedlings continue to grow, you will begin to see the first “true leaves” start to form. These tend to look very different from the cotyledons. Here is a picture of the cotyledons with the true leaves just as they are beginning to emerge:
Once your seedlings have true leaf growth, the plant is actively photosynthesizing. Eventually the cotyledons will wither and fall off and the true leaves take over.
Since I like to use the seed pellets when I first plant my seedlings, I transplant them into a larger container after the plant has established good true leaf growth. I like to use organic potting soil and Cow Pots. There are several types of pots you can transplant your seedings into. Try to choose something that falls apart fairly easily when wet so that you go not have to disturb the plant roots when you transplant them into your garden. Cow Pots are great because not only do they fall apart easily but they also provide fertilizer for your plants. Another great, more affordable option is newspaper. You can form tubes of newspaper to a variety of sizes and close off the bottom by folding it over. If you want to get a precise size each time, you can buy or make a wooded mold for your newspaper pots. Here is a picture of newspaper pot makers sold at Burpee:
They can be a little pricey though so, if you are interested, here is a link to a blog that gives you instructions on how to make your own from PVC pipe at a much more affordable price: http://survivalfarm.wordpress.com/2011/02/06/3-newspaper-pot-maker-designed-and-built-by-cindy-dorfsmith/
However, if the weather is warm and the danger of a frost has past, you can skip the whole transplanting into a pot and just plant your seedlings outside in the garden. There are some plants that can handle a little cold weather too. For example, last year I planted some kale and collard green seedings outdoors. There were several morning frosts but the seedlings survived. Tomatoes, on the other hand, need warm weather and could not handle a frost. I usually wait until mid-May to plant the tomatoes outside. Make sure you know what tolerance your plants have for the cold before you transplant anything outside! In the meantime, make room for your warm weather seedlings under the grow lights in their new, larger containers until the weather warms up a bit more!