I finally received my ggplot2 book and started my learning process of this "new" package. I will keep adding some tips here, so I will a place to look back later.
1. change axis label
p + scale_x_continuous("x axis label") +scale_y_continuous("y axis label")
2. change ticks and labels for axises
p + scale_x_continuous(breaks=seq(1,2.5,by=.5),labels=letters[1:5])