Bought an oldish teenage girls bike wit...

You may be able to fix this quickly if it isn't too dirty. Try spraying WD40 onto the chain, sprocket gears and bearings as you pedal backwards with one hand on the sprayer and the other hand turning the pedals backwards. You may also be able to hold a wire brush with one hand and turn the pedals backwards with the other, and clean things by holding the wire brush slightly against them. If this works, when done, spray a heavier oil onto the parts.