Is it a king keg thing like this???
If so, here's the instructions.
Sterilise the keg and all things you're gonna use with it... (should have got sterilizer as well - did you?)
While thats happening leave the 2 tins in a sink of warm water to soften them up a bit. Boil 6 pints of water and add them to the rinsed keg. Pour both tins in and mix around. Add 29 more pints of cold water and mix around again. Sprinkle the yeast on the top. Tighten the cap and put in warm room for about 3 weeks.
Make sure the rubber on the pressure valve (underneath the lid) covers the little hole in the brass else the co2 comes out - i learnt the hard way!
Course if you have something entirely different then all bets are off - couldn't find "youngs happy brewer" in google...
All the best