Hey,
First of all the thing you need to do is keep calm lol.
As a BestMan you have to on the run up to the weddding remember to go get your suit fitted and make sure groom does to and ushers make sure they know the times and places for these bits.
Of course the stag do which am sure you'll have no problem organizing you've prob already done it in your head if not started looking into it (dont do it 2 days before or the night before because he'll prob still look rough and be lacking in sleep).
On the night before keep the groom at your house or hotel so he doesnt wake up same place as he bride. Take him for a one to one drink to congratulate him just the one (it'll take his mind off the day ahead but Dont get plastered).
On the day make sure you have all the button holes for groom yourself and ushers. Spare black socks as someone ( ushers or groom) will think it doesnt matter to turn up in white when you might be able to see his socks when he walks. Make sure everyone has cufflinks and looks smart. Have the one small Miniture drink to calm his nerves. Talk to the groom and keep him calm. Make sure you know how you are getting to where they are getting married.
When you get to the place make sure the usher knows there jobs. Talk to family members and guests until its time to take your position.
Stand by him at the alta make sure you dont forget the rings.
At the dinner make the speech not to short not to long i'd go for about 5 mins ish. As he's your brother you can add a few comments or stories from when you where young. How he met his now wife. Towards the end Congratulate the bridesmaids onhow lovely they look and what a good job they have done. Say your proud to be standing by your brother on this special day and ask everyone to raise there glasses to mr and mrs .....
Your pretty much free then but you may want to start the daning after the first dance with a bridesmaid as it encouirages others to get up and dance.
If the mr and mrs are getting driven away remember to decorate the car.
sorry this is so long but hope it helps. let me know if there is anything else
xxxx