Geting fit after having baby?

i just had my little boy on 12/26/10. how can i get back in shape? i have a good amount of streach marks and can really leave as during the day its just me and my sister and although she helps me a ton with taking care of my little man its not her child hes mine. so i would feel bad if i just left even to go for a walk or anything. i live in somone eles house and no one can really come over to help watch him as the home onwer wouldent like that and living here for free and whatnot im not going to upset this person. i was 144 beofr i got pregnant and now after having him im 147 and my belly feels squishy and everything.. also any loctions that have helped streach marks look better