You could just go out and buy plants that are already a few months grown, but that can get expensive in a hurry. Like most people I try to do things as cheap as possible. I prefer to think I do things as cheap as reasonable, and the way I will tell you how I do it also recycles some of your waste. It is also cheaper to start with seeds. You could also buy greenhouse kits, but those too can get expensive fast.
I can start my garden for $20 (+tax) and a little time.
-- Seeds, $5
-- Bag of planting dirt, $5
-- Fertilizer, $10
You could save $5 if you used dirt from your yard or garden, but right now everything around me is mud, or frozen mud. its worth the $5 to me to start with some good dirt thats ready to plant. If you planned ahead from last garden season, you may be able to save more money but saving seeds from last years plants. If not, you can always plan to save seeds from this years harvested plants.
Along with those items you need something to put them in. Here is where you can save a lot of money and do some recycling. the big tip is to use empty milk jug. 1 gallon, or 1/2 gallon.
--Knife and or scissors
if you cut all 4 side the top can be saved and set back on top after planting, this makes a mini greenhouse. or you can through the tops in the recycle bin.
if you cut only 3 sides the top of the jug can be set back up after planting, again making a mini greenhouse. this option keeps the top setting on better.
i have done both in the past. my recommendations: if you are only going to be using these in your home remove the top, if they will be setting outside at some time i would use the ones with the top still attached.
i have also used the top as a funnel. it has to be something you're filling that has a larger opening, but it has come in handy as well.
the last step is to mark the planter. you can use a marker to write on the planter, or sometimes i tape the empty package to the outside. just mark them. many plants look the same when they are just beginning to grow, save your self a headache and mark them.
when i transplant the plants i also dump the dirt in the garden. you loose soil every year from wind and being tracked out on shoes and animals. after transplanting you may need to add more fertilizer, read the labels on the fertilizers. after the planter is empty i rinse them out and put them away, keep these and use them again for many years. this keeps them out of a landfill, and you are recycling or reusing a waste item in your own home.
the top pic is a 1 gallon jug planter, the bottom with 2 planters were made from 1/2 gallon jugs.