Your height: 4'5" to 4'9" (135 to 146cm) approximatly (?).
Frame size = 18in/46cm
Comments:
Mountain Bikes: they are sturdy and designed to go everywhere including off-road. Good for rough riding. In general heavy and with deep thread tires slow on paved roads. Not good city bikes unless narrower and slick tires are installed.
18 speed: perfect to go everywhere including going up very steep hills. There are 3 front chain rings and 6 rear gears. - Derailleur:
Tires 26 x 1.95: wide tires, deep thread, all terrain.