You robot, suprisingly, is only as good as what it is made out of. Most of the competitions involve contact, either with a goal or with another robot. Although FIRST is not at all like Battle Bots, if you build your bot out of wood, it ain't gonna last very long. There are only a few types of materials that you can use on your robot. I'll run through each one, and tell you their pros and cons.
Aluminum
By far the most common element on robots, aluminum has the advantages of being extremely durable, flexable, and still very light. Aluminum comes in many different configurations, such as thin sheets of aluminum, thick angles, and pipes. Although it is a bit more expensive than some other items, it is what you should make the majority of your robot out of.
Steel
This is a very heavy, very strong metal. It is a hard material to make a FIRST robot out of due to a few problems: 1: It is hard to drill through or cut easily, and 2: It is very heavy, and hits the 130 mark quite easily. However, there is nothing quite as strong as a robot made of iron, and welding can be used to put it together. Although not the material of choice, small robots made for pushing are very strong if made with this.
Plastic
Although there are several kinds of plastic that can be used on robots, they usually have these advantages: Thin, lightweight, and see-through. The last property is a reason why most robots use this to protect their semi-vital areas. Although it is not the most durable of materials, it can be useful to protect any area that the controllers on the field need to see. Price is a bit of an issue, but it still, if affordable, it is a useful material to be used in select locations. Some forms of it are actually quite durable, but are that way due to flexability.
Wood
This is not a material most teams use. At all. Wood is not flexible or durable, and can break it whacked at hard. Also, it just doesn't look that cool. However, there are some strong points about it. For one, few things are lighter than wood. If you have to add something at the last minute, and durability isn't much of an issue, wood is a good option. Also, wood is dirt cheap. Compared to everything else, a block of wood goes for nothing. And it is easy to cut and drill. However, there are few areas of the robot that don't need a bit of durability. Most things will get whacked pretty hard, so be careful when you use wood.
Game Time!
Here is another one of my poor excuses for games. As always, shockwave is required to play it (lucky shockwave-less people can just go to another page).
