Let me first say, shooting people is a far cry from flowers, rivers and rocks. I speak from experience. You don't need to talk to a flower and get them to loosen up and smile in order to make a good picture. Flowers are generally pretty by nature (I'll stay away from the rocks for now).
As Gonzo said, look into how things are done. I know if I were ever to go into senior photography, I would look a lot into poses used and different ideas (areas to go around town etc). Speaking as someone who has had their senior portraits done, remember that they typically are meant to capture the end of ones secondary education. What I mean by this, is that when they look back they should not only be able to look back and see 'just a portrait' of themselves that looks good, but it should emulate who they are at that moment (what they did, what they liked, hobbies, school activities, who their friends may have been etc.). And lastly, as someone who is providing a service, remember that sometimes you're going to have to abide by what your client wants. Further, in providing a 'professional' service a lot of people may expect that you know exactly what you're doing--so make sure you do.
As for equipment, I would look into something like a 40/50d, or an xti xsi or t1i. They're good introductory cameras (sorry I didn't suggest Nikon, I am a little biased). Other than that a lot of it is going to depend on the style you develop. Do you want to do all natural lighting or do you want to have artificial? Are you going to do studio shots? Where are going to put a studio? When you get into everything it can become expensive, camera here, lens there, lighting equipment there, advertisement, time, travel, etc. But you need to get the logistics down first.
And what do you know about the functioning of a DSLR camera? Eg the correlation between f/stops, shutter speed, ISO?
Lastly, what year are you, if I may ask?