As soon as the ground can be worked, I typically do a simple hand test to see if the soil is ready for planting. I take a handful of soil and squeeze it tightly into a ball. If the soil breaks apart loosely, it is sufficient to start preparing for planting. If it remains in a ball, I will wait a week or so and try again. Hard, compacted soil will not allow your plants proper drainage, which can led to various plant diseases and stunted growth.
Every year, I send a soil sample to my local county extension office to see what type of amendments I need to add to my soil (in the past, I have experienced drainage problems due to heavy clay). I would suggest you send your sample in as early as possible so you can receive your results in a reasonable amount of time. You can also opt for a D.I.Y. soil testing kit. They should be available at your local home and garden store. The results of your test will let you know exactly what amendments you need (if any).
A few of the most common amendments are:
Sphagnum peat moss
I usually work amendments into the soil by digging with a garden fork (I love earth worms and I do want to hurt or disturb them). However, you do have the option of tilling, the above recommendations are optional, but your garden will thank you in the end by producing beautiful and healthy plants throughout the growing season.