Ok, so yesterday I blabbered on about sunlight.
In the immortal words from Fast Times at Ridgemount High: Learn it, know it, live it.
While you're monitoring the light situation, you can get better acquainted with your soil. As far as soil goes, it's important to know if it's acidic/alkaline or well-draining/moisture-retentive. If you container garden, you're in luck because you have the ultimate control over your soil and can perfectly adapt it to the plants you want to grow! Bagged, soil-less potting mixes tend to be slightly acidic, in general.
If you're planting in the ground you need to determine whether it's acidic or alkaline, you need to perform a soil test for pH. You can buy kits or you can send it off to your nearby extension office (they will do a more accurate, more thorough test and it's usually under $10!). A pH of 7 is "neutral." A pH less than 7 is "acidic" (to varying degrees) and a pH above 7 is "alkaline." Acidic is not necessarily better than alkaline for a garden. It all really depends on what you want to grow (although most plants prefer a pH between 6.2-7.2). Which is why it's handy to know what kind of soil you have.
Next, you need to evaluate how well your soil drains. Like acidic vs. alkaline, it's not necessarily better to have well-draining soil than moisture-retentive soil. It really boils down to the plants you'll ultimately select. Here's how to test the drainage of your soil: http://www.ehow.com/how_4543171_test-soil-drainage.html. If you're using a bagged, soil-less potting mix, it's generally considered well-draining, but make sure your container has adequate drainage holes!
Armed with this information you can either amend your soil to fit the needs of the plants you want to grow or go with the flow. I have, personally, have chosen a hybrid approach: I have worked compost into my moisture-retentive clay soil to help it drain better but have not changed the pH (mine soil is around 6.0-6.5). If you decide to go the amending route, take all the information you've collected to your nearest reputable nursery & ask an expert what you need to do.
If you're making a perennial bed, I would also suggest doing a NPK (nitrogen, phosphorous, potassium) test (which if you're getting a soil test from your extension office they will automatically do or you can get a home kit for it). This makes sure that you have adequate levels of nitrogen, phosphorus and potassium in the soil. If your levels are inadequate, your plants won't have all the nutrients they need to grow. This is important if you're planting perennials because you want them to last and won't be planting every year. It's best and easiest in the long-haul (or so I'm told) to just get everything right from the start. If you have inadequate levels, amend your soil (ask your local reputable nursery what you need to do).
Tomorrow? The final part of this triology: Plant selection.
Are you stoked yet?!?