One of the best ways to improve your diet is to keep a journal or otherwise track what you eat, compare it to your body's needs, and viva la difference. There are a growing number of very good tools to help you assess the basics of your nutrition status and then to track what you eat and your activity.
Here are some good ones:
My Pyramid Tracker - This is a easy to use site that will help you compare your diet to the Food Guide Pyramid. While many knock the Pyramid, just over 1% of American actually eat this well and get adequate physical activity.
NHLBI Menu Planner - A free and simple to use tool for menu planning. Basic information sheets from the National Heart Lung & Blood Institute.
Ask the dietitian healthy body calculator - This is the best free online nutrition assessment tool I've seen. Provides a very good estimate of calories and other needs.
Meals Matter - Meal planning and other tools brought to you by the Dairy Council.
Spark People - A free community with meal planners, diet trackers, personal blogs and other aids to get your eating in shape.
FitDay - This one's my favorite because it gives detailed nutrition information. You can track specific vitamins and minerals as well as other nutrition, activity, mood and other information. Easy to use too. Free.