IMHO, I wouldn't worry about buying anything else. Start out from what you have there, then you will soon find out what's it is capable, and what's not. This would be a better way to learn, then just go out and buy them all!
Yet, I still recommend buying a 50 f/1.8 lens if budget allows. That can give you a better idea what large aperture can do with your DSLR vs P&S.
And next to buy, EXTENAL FLASH .. in order to let you play a little bit about lighting ..
Then as EL said, 50mm 1.8 is a great lens for pratrice. ( I am big fan for prime )
^_^
( by the way, 400D? so you have it for couple years ? I have 400D too, great camera, easy to use, good IQ as well! I still use the 400D+kit lens a lots)
I did start from my 400d kit
after a couple months, I bought a cheap cheap lense for telephoto ($150 on craigslist). Then a year later I got my cheap but fun prime lense 50mm F1.8. That's all I have spent on my camera LOL and I still have yet to get an external flash +w+
filter is good idea ... but dont buy those cheap one, make sure get MC ( multi coated )
However, filter more or less just for protection.
Um .. I guess I am old school la.. still prefer and enjoy doing more stuff WHEN TAKING the picture ... (or I dont know PS much la.. ) I just use Raw Editior for adjust sharpness, brightness, and fixing white balance etc ..
Um .. you dont really need hood for the kit lens .. (遮光罩) of course, it won't hurt, and it may hlep in some case, but most the time, not much different, and a new 遮光罩 is like $30 plus ..