pretty much what others said.. be there "at ready".
see that everything goes smoothly, so that he or his other malefriends dont make or get to into too much trouble. (in bachelor party and during the big day.. the ceremony and even in the after party.) you kinda need to be the "big brother" looking after and taking care of the husband so everything works out. get him ready on time and keep him calm and at ease that things are working out just fine (thats what the maid of honour and mothers worry on the bride side).
so basically you need to stay sober enough to be in control of things. just be prepared of small mishaps. so that the happy couple dont have to worry about a thing. you get him or them everywhere in time and make sure they have all things needed (tie is right and tux is clean and the rings are available).
like in my friends weddings.. the best man (who ever he was) knew whom could be helpful when the fuses went out of the place.. who has lights and tools.. whom is sober to drive and pick up something in last moment.. where to park the car/limo when the couple was going to get their photos taken.. whom are good at opening up a locked doors or fixing broken things.. whom he needs to babysit away from the punch and other drinks until granma has gone home.. who might cause a ruckus and see the fight coming early enough to usher them outside away from everyone else.. helps to know yourself and your friends or families strengths.. you know who to ask for help and who to watch out so they dont do anything stupid.
about the speech, ask if they (groom/family) asks you to make one or not.. keep it short, keep it simple. talk why he is your best friend or why you are thebest man and briefly talk about your and grooms history, praise his good qualities and hers too. mention how he found the best girl for him in the world. and how rare it really is that two people are in sync. etc
or talk to maid of honour of things the bride might want to be mentioned. (like: things how they met.. sometimes the story has different side to what you heard.. he said he "picked her up out of the crowd on the moment she arrived" and she says that she had been chasin him for weeks until he was too drunk to resist.. she picked him out not the other way around .. he just didnt know it. hah)
this might soudn like overkill and cause too much stress of how to handle the job.. i dont mean it so. i just try to give hints of things that might go wrong so you might be prepared to handle them. if you are smooth operator, then the operation will go smoothly.
there will be a momentum shift in the day when you see the stress of all participants go away and things mellow down.. thats the moment when you can also start loosening your tie and really enjoy the day..
it will likely be hard to handle your own emotions during the day, but try your best.. if you dont feel like you are up to the task.. tell the groom about it beforehand so he knows to find someone else.. or if during the day you will get a weak moment, try to have a back-up friend there near by to take up the slack while you gather yourself back in order.
in one of my friends weddings, the groom had 2 bestmans just because of this kind of reasoning.. other one had just married his long time girlfriend few months before, and she was diagnozed cancer just a days before wedding.. (she is doing ok now)
anyways.. hope it all goes ok with you.