The first step to building a bot is to think about what kind you would want to build. When I start the project I always take a look at what people have done already and draw from the knowledge learned by others over time.
A good place to start with your research is the builders database. http://www.buildersdb.com
this website is used by most competitions for registration. One of the requirements of this site is each team/robot have a profile with a picture of their bots. Because of this you can easily browse hundreds of other robots in your weight class.
Anther good starting point is to determine how much money you are willing to invest. Unless you have lots of parts hanging around which can be re-used from other projects you will need to account for ever item from motors to materials and don't forget about the machining/ building time. Below is a list of the components commonly required for most combat robots.
The main reason that setting a budget is important for your project is that you can very easily spend hundreds if not thousands of dollars very quickly. Robotics is a fun hobby and can fit any budget if you plan for it. The last thing anyone wants is to get part of the way into the build and then not be able to finish due to funds.
*Drive motors/ transmissions
*speed controllers for each motor
*radio control system (receiver and transmitter)
*main power switch
*shafts and axles
*screws and fasteners
*weapon (material or purchase)
It is also important not to forget spare parts, as during combat you will break parts and components. Also having at least 2 sets of batteries will be necessary for competition