I just started my period yesterday after not having it for 8 months. I lost 23 pounds since December (im 5'8) and in the past 2 months have been working to put my weight back on.... I gained about 15 pounds back and finally started my period (now at 140). The reason your not getting your period girl is because you do not have enough body fat. If you are underweight you usually will not have a regular menstrual cycle. Dont worry once you put a lil more weight on it will start right up! I was extremely concerned to but sure enough after putting my weight back on it started!
as for what foods to eat to gain weight its to each their own, but dont go down the fast food road. Not only is fast food garbage and taste terrible, its just unhealthy, has to much saturated fat and grease and puts weight and cellulite in all the wrong areas. There is nothing better than home made meals OMG