Be very careful. The worst thing I ever did was get a kitten when I had a ten year old male. Despite following all the advice about how to integrate the two (scent swapping, separate litter trays etc etc) it was a nightmare. The old boy really was not happy and I felt very, very guilty. I would never do it again.
If you do go ahead, it's crucial the older one has his own space - somewhere the little ones wont hassle him and where he can eat and sleep in peace. (Our kitten thought nothing of biting the older one's tail when he was eating, for example; RSPCA and Cats Protection offer good advice on this). Make sure you don't make a big fuss of the new one in front of the older one too - they really can get jealous.
The kitten/s will initially need their own room. They should not be left alone for too long (certainly not a whole day). It's also important they are allowed to sleep, so if you have young DC, they will need to be watched.
Having two, in many ways, is better than one as they will keep each other company and hopefully not upset the older one so much (well, in theory). We once had two young boys who got on brilliantly, but others have not been as lucky. We were recommended to get one of each.
I know getting kittens is exciting - but do think carefully. I ended up in tears when I realised how much we'd upset our old boy (who was actually a laid back cat) and wished I'd never done it.