When you do introduce it, just give a tiny piece, as big as a postage stamp, on day one, then nothing on day 2, then another tiny piece on day 3, and so on, gradually building up the portion size over 2 weeks. This gives the digestive bacteria time to build up sufficiently.
Only introduce one type of new veg at a time, and preferably don't feed fruit at all. I give mine a small chunk of carrot (needs to be only a small amount per day as it's sugary) a tiny piece of brocolli, just for a treat as it can cause wind but they love it - and mainly spring greens, which you can get at major supermarkets - they have about a medium sized leaf a day.