Negative ease is what you do to make an item have to stretch to fit... like you would with a hat or gloves. With a sweater personally I don't like negative ease... I prefer regular ease, which is making the item an inch or two larger than your measurements so it's not body-hugging.
To increase the size, would depend on whether you need to increase the entire size, or just the bust size. In which case a couple extra increases in your increase row across the front should take care of it. In reading the pattern, you should have increase rows for the front when you reach the bust line. This may take some frogging and redoing to get right. Measure across your bust from side seam to side seam, and that's the measurement you're aiming for (you are not aiming for 1/2 the full measurement, that will pull your side seams to the front). When you do your increases, you'll need to measure as you go, and if you don't hit the correct measurement, frog it and do more increases.
Without seeing the pattern, it's hard to tell you exactly how to go about it, but that should give you a good idea.