a robot that can go up and down stairs, and get things for me.
This is no easy task, in any sense of the word. Very few people have been able
to build robots that can successfully go up/down stairs. Probably the most successful
is packbot by irobot.com. It's fairly large and has tracks.
Then, to "get things for me", is another very difficult task. This means manipulators,
claws, possibly computer vision, on and on. You'll probably also want a control GUI
for your PC, and an RF link.
From the idea you've presented of your level of robotic knowledge, I'd say you'll
want to build another 3 or 4 bots with increasing levels of complexity, before
you're ready to achieve your stated goals.
It's all one big learning curve. Each robot provides more knowledge you can
bootstrap off of. This is a non-trivial amount of work you have in front of you.
You might also want to go ask around for more info on a dedicated robotics
forum, like SRS.