If your thinking of concreting an area you have to prepare that area as follows

 Step1 Mark out the area with a paint marker to show the out lined areas

Remove all grass and top soil Ect from that area as concrete and grass don't mix and can cause voids under your slab

Step 2 make sure the level that you want your concret finished at and the level after cleaning away grasses that you have a depth of more than 75 mm it might be more depending on the type of concrete you are using and the areas you are concreting

For drive ways a min of 125 mm is required

For general paths a min of75 mm is required so make sure you have the clearance for the type of concrete you are installing

A 20 mm crush rock is required for the base of you concrete to help with bonding and compaction of the ground beneath to give a level and firm area a compactor or water wheel is used to get a firm base

Do you require drainage for you concreted area seek advice on this

Step 3 A mesh sheat is required to. Hold your concrete together a size of f 72 is the normal size I use on most of my jobs

Cut the sheet to fit inside the area you want concreted 

Box a a perimeter around the area you want concreted out of timber and fix into place with pickets

Step 4To find the volume of concrete you need To Measure the length by the width and multiply It  and This will give you the volume of concrete you need

Eg 1200 mm x1200 mm X.100 mm =.25 mtrs of concrete

You will have sorce out your local concrete plant for delivery