Another seed starting method that can compliment or be used in place of the Ghetto Greenhouse is the "Baggie Method" if you don't have the space or desire to make seed starters out of plastic soda bottles. This method also saves you money because you don't need soil right away to start and when time comes to transplant your seedlings you can put them in an appropriate size pot. You only need a plastic sandwich bag, some seeds and a paper towel.
The kinds of seeds you can start with this method is really only limited by the variety in your collection or what you can find for sale. In this example I started citrus seeds from a citrus I got from the grocery store.
Once I was done eating this particular citrus I saved the seeds and soaked them for a day or so in a glass of water. When I was ready to sow them using the baggie method I laid them out on a napkin and moistened the napkin with a spray bottle. You want to moisten and not soak your paper napkin to avoid having your seeds get moldy. It's imperative if you're going to start citrus seeds that you sow them immediately because the longer you wait the less your germination ratio will be.
Once I had soaked my citrus seeds overnight and set them on the paper towel which I moistened I folded the napkin in half and set it inside the plastic sandwich bag which I sealed. Then I found a very warm spot for them in my bedroom. Since it's winter I take advantage of the heater we have running and place my seed baggies near a vent. The warmth from the heating vent helps with germination especially in the middle of winter when temps aren't really optimal for seed starting.
After a few days days I checked on my seeds and found that some had started to germinate. Once they get big enough I pot them up into little pots and let them continue to grow until they're big enough to re-pot again. Citrus like a lot of light if they're going to be grown indoors and whenever possible should be allowed to spend Spring and Summer outside.
At this point I should tell you that it will be a number of years before your citrus tree gets big enough for it to flower and fruit. But if you're a patient person or just growing for the fun of it that shouldn't be a problem for you. You can use this method to start a whole collection of citrus trees from seed to grow in your home or yard. Next time you're in the grocery store look for Key Limes, Calamondin Oranges, Kumquats, Mandarins and give them a try. If your store doesn't have a large variety of citrus available check out some of the ethnic grocery stores in your areas and discover a whole new world of fruits and veggies.
If you're not interested in growing citrus from seeds then you can use this method to start any kind of seeds that you are interested in. But I'd suggest sticking with larger seeds because they'll be easier to pick out and transplant into pots when they've sprouted. If you find that your bag retains a lot of moisture while you're waiting for your seeds to sprout open it for a few hours a day and let a little of the excess moisture evaporate.