We got the minibot from FIRST. Are there any instructions on how to build the minibot.

What I assume you got was the FTC kit from FIRST choice. You can use all the pieces from that, as well as a few additional things (A full list can be found in the game manual), to construct the minibot in any way you want to. I suggest re-reading through that part of the manual in detail.

Well its more like limitations and criteria you must meet. My team managed building a prototype mostly using everything provided with the FRC kit. I forgot how my team got it.

In the manual is a list of material you are allowed to use, there is a link in the manual pointing to a parts list (including pictures) of the TETRIX kit defined in the material list.

Otherwise you should come up with your own effective design. Try to aim for a minibot that can at the least go up in less than 3-4 seconds.