You can find the answer to this question in Chapter Nine, which includes the lead-up to the rumble and the rumble itself. Ponyboy tells us that as he and his brothers leave their house and make their way to the place where the rumble is going to happen, Two-Bit imitates a Soc and, playing this part, is interrogated by Ponyboy and the others as to what he does for fun. Note how Ponyboy describes Two-Bit in this section:
Two-Bit was the only one wearing a jacket; he had a couple of cans of beer stuffed in it. He always gets high before a rumble. Before anything else, too, come to think of it. I shook my head. I'd hate to see the day when I had to get my nerve from a can.
Two-Bit therefore prepares himself for fights by getting drunk, as Ponyboy puts it, getting his "nerve from a can." Note how this indicates the fragility and insecurity of Two-Bit himself by the way that he has to turn to alcohol to give him the courage and determination he needs to participate in such urban brawls.