keeping weight on

Hi iam a 34 yr old mother of 3 and was hoping in my 30s my metablisim would slow down but no luck. I say this because Ive been 100lbs since I was a teen. I feel like I look great, not bad for a 34 yr old but I feel tired and sometimes spurts of energy then sluggish. I have a new addition to the family my son who is 2 yrs old so I have lots of fun running around with him. I am about 5'6 my mother is the same way. What foods should I eat but stay healthy? I guess bottom line is HOW DO I GAIN WEIGHT IN A HEALTHY WAY AND KEEP IT ON. HELP!!

Thanks
Ann