The key with potted plants is to start small and gradually get bigger. Planting seeds directly into one-gallon pots has worked out horribly for me. What I have found to work wonderfully is starting seeds in ice-cube trays. Head out to goodwill and buy a few old ice-cube trays, then fill them with soil and put a couple seeds into each compartment.
Once the plants grow too big for the ice cube tray, put them into something slightly bigger. If you cut a .5 liter water bottle in half, then re-pot each section from the ice-cube tray into the watter bottle bottom, it's usually exactly the right size progression. And, since most water bottles are clear, you can see when the roots reach the bottom, and it's time to re-pot.
Once the root ball fills the bottom of a water bottle, it's big enough to put into a one gallon pot.
With a few types of plants (green beans or tomatoes work the best) making an upside-down hanging planter can maximize your space. Here's how it works:
1. Take a piece of cotton around 3"x3" and cut a slit in it.
2. Take a plant from the ice cube tray and gently push it through the hole so that the plant is on one side of the fabric and the roots are on the other side. Whip stitch the slit in the fabric closed .
3. Carefully feed the roots into the neck of a plastic bottle and secure the fabric around the neck with rubber bands. You may want to put the rubber bands on the bottle first, then feed the plant through, then secure the fabric, because you're less likely to hurt the plant this way.
4. Flip the bottle upside down and carefully cut out the bottom.
5. Fill the bottle up most of the way with potting soil.
6. Poke holes near the top of the bottle and string some cord or embroidery floss through them.
7. Find a nice place in the window to hang your plant!
Note: If you're not a big fan of the recycled-chic look, you can decorate the plastic bottle before hand with sharpies, nail polish, fabric, or anything else you want.