An easy way to start building your first robot is by using modules. A couple of tips first though, you don't need expensive materials and tools, its amazing what you can make with a few odd scraps and pieces of junk. Have a look round the £1 shops, some of the cheap toys might already have a lot of what your looking for in them. A classic case for this is for a motorised base - try using a stripped down remote controlled car, or any toy that moves around on its own.