I think a good vitamin would be a great place to start. My daily vitamin has Vitex in it, and lots of other good stuff too, and I definately notice my energy is lower when I'm not taking it.
Have you charted your basal body temps and fertility signs to see what your cycle looks like? I found that information very empowering, there's nothing like the first time you see the evidence that you are actually ovulating!! Now that I know my cycle is normal, I have dropped the charting because it has created too much pressure for us to 'perform' when we know I'm fertile
It may or may not create that pressure for you, I wouldn't do it if you start to feel too obsessed!!!
It's so hard to take good care of yourself when you have a little baby you are caring for 24/7. Maybe just simple things like making a point to get outside for a walk with your baby every day the weather permits, or take a yoga class at a time when you can leave your DS with your husband. I've also found that some of the yoga videos are very good, especially the 20-30 minute ones you can do while your baby is sleeping. You can also do some light weightlifting at home to get a little exercise and build some muscle.
I don't know what kind of food you are eating, but I think a healthy diet all starts with your grocery list. You have to be able to reach in the pantry and grab something good for you without thinking about it. If you are ok with dairy products, I have found that one of those small organic yogurts they make for babies are a great, easy snack, and the only thing you mess up is a spoon! Maybe a nutrition book, or check out the Brewer diet that is recommended for pregnant women, it's high in protein and really healthy, maybe you could adapt that to fit your needs.
I think the most important thing is that you really want to make some changes, if you are motivated, that's half the battle!!!