in transformers jargon an "alt-mode", or alternate mode just means the vehicle that the robot transforms into. For example, Optimus Prime's "alt mode" is his truck, the mode he converts to from his robot mode. This is the most general definition, but you can check out wiki for something more detailed:
http://tfwiki.net/wiki/Alternate_modere-reading your post, im guessing you are asking about Heavy Metal War(in which case this post could be in the wrong forum i think, so it might need abit of a relocation). In that case, in HMW you can choose what your dude transforms the moment u create your robot. You should see a whole truck load of icons right after u create your character, any one of which could be your "alt mode". Different alt-mods give you different abilities, but i'll leave it to you to experiment.