How pg are you? I have a 21 month gap between DS1 and DS2 and then a 23 month gap between DS2 and DD.
DS1 and DS2 are 4.5 and almost 3 now - they are constant playmates - from the moment they wake up until they fall asleep (often still calling out to each other through the bedroom wall) - running around giggling, shouting (arguing too, of course), making up games together. It is fab.
The first year is the hardest because both of them need constant attention. From the point the younger one is about 18 months it gets hugely easier as they entertain each other.
I would concentrate on making it as positive as possible for your older one. We bought 'big brother' books, talked about 'your baby' (rather than 'our' baby), introduced the baby to other people as 'DS1's little brother'. Also, when DS2 was born, grandparents were around to make a big fuss of DS1. And DH had quite a bit of paternity leave/holiday and was able to spend a lot of time with DS1 (and/or enable me to spend one-to-one time with DS1 too). All of that helped.
I really can't describe to you how wonderful it is having my little brood of three, all very close together. My SIL (who also has 3, but more spaced out) keeps telling me how lucky I am having them so close together because they all like the same activities - no having to drag an unwilling older child around a toddler-play-place etc. It's true, we pack them all in the back of the car or strap them all on the pram (DS1 rides on the footplate...) and go off to some very tame activity like the park, and they're all delighted with it...
It is obviously hard work too, very sleep-deprived and there is always a job to be done. But I feel like I'm getting all those hard-work early years done at once, instead of spacing them over several years (that's what I tell myself anyway).
Good luck. You are blessed, really.