First off, you've picked an excellent camera to start your journey into DSLR photography. The A200 is a camera with a great set of features.
To the necessities,
- Read - Know your camera. The better you understand your camera the better your photographs will be, it's that plain and simple. Photography is about capturing moments & if you know how your camera operates that is one less obstacle in your way of capturing that moment.
- Join a photography specific forum. TFP's a great forum but, there you'll have a multitude of peers that have the same equipment as you & their experience to benefit from. I'm a member of a couple of different forums & I can't tell you how much I've learned from the discussions there.
- I'm going to assume that you shoot in RAW. If you don't already, start making the transition by shooting in RAW+JPEG. You will have a much greater flexibility with your photographs in post-processing & you'll be much happier that you did down the line.
- A good bag - One I can recommend would be a
or a comfortable backpack (if you do a lot of hiking)
- A comfortable Neck Strap. It's such a small thing cost-wise but it has been an investment that has paid dividends many times over for me.
- Extra batteries or maybe the battery grip accessory later on.
- Lots of storage, and instead of buying one 8gb card go for two 4gb cards. That way if one of the cards fails while you're out you will still be able to shoot and you only (potentially) lost 50% of what you've already shot.
- Filters - Ultraviolet to start and then Neutral Density and Polarizing as you need them.
- Tripod if you're doing any type of long exposure shots.
Have fun out there & be sure to post some of your shots here. I'd like to see what you find out there with your camera.
If you want any reading materials relating to digital photography, PM me & I'll hook you up with a zip of all the e-books I've got on the subject.