Right now it's too hot to plant, so you could use the time to weed. Start with a small area -- the winter is a great time for garden clean up and planning. You have found garden-web, a good resource. Try local garden clubs. What do you want to grow: flowers, veggies? annuals, perennials? Daylilies are good b/c you can take them with you when you move. Good luck! Enjoy your piece of (rented) earth! (Aren't we ALL renters, in the end?)

I also live in DC, in the Mt. Pleasant neighborhood. Urban soil can be terrible to garden on so you have my sympathies! What part of the city do you live in?

I agree, there's not a whole lot you can do now. I do have some questions--what exposure do you have (i.e., sun vs. shade) and how long do you expect to live there? In the short term your best bet is to try to get the weeds under control and plant some bulbs and/or tough, easy perennials this fall to provide some color next spring. In the spring, you can either plant more perennials (if you expect to be there a while) or some tough, easy annuals.

You might also consider container gardening--that way it's easy to take the plants with you when you move.