In the bottom corner of the fridge you should have a small plastic viewing hole, make sure that the 12 volt supply is on and turn the control knob on the fridge to the gas setting, press the control knob in & count to 8 then press the ignition button in quick succession whilst holding the control knob in, you should see a spark in the viewing hole and the gas should ignite, once the gas is lit keep the control knob held in for a count of 10 (slowly) then let go of the control knob & the fridge should keep running on gas, you will see the flame in the viewing hole. The control knob has 4 positions off, 12 volts, gas & mains electricity 12 volt is for use when towing only, you will need to have a 12S socket with a grey lid on your car, the gas & mains positions are self explanatory.
Propane cylinders (orange) are best for use with the caravan as it continues to gas down to around -40c but butane (blue) stops gassing at around 1c which will make you pretty miserable & cold.
I would personally get the caravan serviced there are some mobile service companies about and they will check all the running gear, gas and electric systems and will be qualified to do so.
Can I also point out that your caravan when loaded to its. Maximum allowed weight shouldn't weigh the same or more than your car it should be around 85% of your cars kerbweight, as you need a hefty car to prevent the caravan starting to weave about when on the road, a huge twin axle caravan being towed by an Astra or Focus sized car is a disaster waiting to happen. HTH