The 50:1 tag is for the gas oil ratio. You should add 50 parts gasoline to one part oil. Here in the US this equals 1 gallon of gasoline to 2.6 ounces of oil.
To tighten your chain loosen the 2 nuts on the side that hold the bar tight to the saw. Then look on the side of the bar on the engine side. There should be a flat head screw that when turned moves a small tit that in turn slides the bar forward and backward. Turn that screw to the right and the bar and chain will tighten up. All that is left to do is to tighten the 2 mounting nuts after that