You tube has a million videos on laying concrete slabs. The main thing is to decide how much weight it will take. If a vehicle will drive on it you need steel reinforcement. If not, aim for at least 100 mm for light foot traffic. You can buy the gravel / sand mix bulk and add concrete as you go. Mixers are defineately the go. You get what you pay for so try Tradetools at Caboolture. All their tools are heaps cheaper than Bunnings/Masters and that is for the same brand. Make sure you use a bucket to measure. Shovels are not accurate. Don't add too much water. The more water the weaker mix. Put builders plastic down and then either cover when laid or wet regularly with a hose. Concrete sets by a chemical reaction, not drying out.