Step 1: Designing: Deciding
Since this is a delicate and fairly expensive project doing lots of designing and background research is good cause it might just save you some trouble. First start with the overall function/goal and structure/looks, if your not sure what to make or what it should look like look at robots made by other people, try to find a problem that your robot would be able to solve or ask people nearby for inspiration. With stifiam I found the old picture above I had drawn when I was young and decided I would try to make stifiam look like him and he would be self charging when needed, had emotions and he could make sounds. I also decided he would be standing on some mobile platform so that he could move, his height would be as big as a cat and bigger than his width .After deciding the goal and looks lets divide the robot into parts so that its easy to organize and build. You will most likely have a sensors, motors and some sort of controller that's why I like to divide my robot into sensing, moving and controlling.
Step 2: Designing: Sensing
Lets start with the sensing part. In order for the robot to be able to interact with its environment it will need to be able to sense its environment . There are many types sensors in the world like light, motion, temperature, sound, distance ,pressure, etc. sensors. If your robot is moving you probably want a distance sensor so that when your robot is too close to something it can carry out a decision or you can control how fast it is going depending on how far it is from another object. I decided to use Parallax Inc.'s PING))) Ultrasonic Distance Sensor for distance measurement cause ultrasonic measurement is not affected by light, the ping))) sensor only uses one pin plus I have some experience using it and I haven't encountered any problems using it.
Step 3: Designing: Moving
Depending on what type of robot your building and what you want it to do you will probably want some movement, the best way I know of is dc motors,stepper motors, geared motors, solenoids and servos. If you are new to motors here is a awesome instructables by audreyobscura.Motors are pretty much everywhere. If you have a old cd player, old robot toy or a RC car you'll probably find some dc motors but servos are less likely to be around you, I find, without buying some online or at a shop. I wanted stifiam to have two moving arms, some sort of mobility on the ground with wheel, a moving neck.