Hi Maggie, sounds like you are starting a wonderful project. There is a community garden in Oceanside CA that is used by apartment dwellers and people with special needs. It is pretty cool. They rent out the plots (all raised beds) and have come up with pretty nice designs for wheel chair access.
Here is the basic bed prep program from Howard's main site:
BUILDING NEW BEDS - Scrape away existing grass and weeds, add 4-6” of compost, lava sand at 40-80 lbs., organic fertilizer at 20 lbs., wheat/corn/molasses amendment at 30 lbs./1,000 sq. ft. and tilling to a depth of 3” into the native soil. Excavation of natural soil and additional ingredients such as concrete sand, peat moss, foreign soil and pine bark should not be used. Additional compost is needed for shrubs and flowers than for groundcover. Add Texas greensand to black and white soils and high-calcium lime to acid soils. Decomposed granite and zeolite are effective for all soils.
FERTILIZING – Broadcast organic fertilizer 2-3 times per year. Foliar feed all plants during the growing season, at least monthly with compost tea or Garrett Juice. Add volcanic rock such as lava sand at 40-80 lbs./1,000 sq. ft. Use dry molasses and humate the first few years. The rate can vary from 50 lbs/acre to 200 lbs/acre.
MULCHING - Mulch all shrubs, trees and ground cover and food crops with 2-5” of shredded native tree trimmings to protect the soil from sunlight, wind and rain, inhibit weed germination, decrease watering needs and mediate soil temperature. Native cedar is the best choice. Other natural mulches can be used but avoid Bermudagrass hay because of herbicide residue.
WATERING – Water only as needed. The organic program will reduce the frequency and volume needed. Add a tablespoon of apple cider vinegar per gallon when watering pots. Use 1 ounce of liquid humate in acid soils.
Read the sticky notes and FAQ's at the top of the different forums and you will get a great deal of information. You can also do a search on the Forum or the Dirt Doctor site for specific questions.
Good luck in your garden
Looking forward to hearing about your progress.