I Need To Lose Weight?

ok so i am a 15 year old girl. i am 5'3" and 345 pounds. both my parents work from 7:00 am to 7:30 pm from monday-saturday, so they work all the time so its just me and my 6 month old baby sister in our apartment for most of the day (the person who lives next door to me takes care of my sister while im at school), and my boyfriend likes to come over (he lives right across from me). i am always taking care of my sister, i change her diaper, i feed her, i put her formula in her bottle and so many other things. beleive me i love my sister its just she can be a hand full sometimes. and with taking care of my sister i get stressed out and i when i get stressed i eat and thats why i eat so much and with school i get more stressed. i have decided to lose weight cause i am just sick of being fat and everytime im with my BF i feel very insecure beause hes like 6'0 tall and only like 140 pounds. as for now i cant do much excersize i can only walk a few feet until i have to take a break. i am going to start walking with my boyfriend and my sister in her stroller to build up my leg strength till i can walk normally.
what im asking is
what excersizes can i do when i can walk normally?
and how can i stop stress eating?