Push Ups and Sit Ups?

Ok, I'm going to the beach this summer and really need to get my body defined. All i can do at home is push ups and sit ups. I weigh 117 lbs and am 5' 11" tall. If i do 3 sets of 15 pushups and 3 sets of 50 situps will that help show abs and make my arms a little bit bigger? the other thing is that, believe it or not, i have a small amount of stomach fat (you can see I have rolls) and i have NO pecs whatsoever. Can i still acquire abs and bigger arms, possible with small pecs, and how long will it take? Also, do i need to do more then what I am already doing? THANKS A LOT FOR THE HELP!