you put a peg in the middle with a piece of string the right length and lay your bricks at the end of it all the way round . get a small spirit level and lay the first brick at the highest level with not too much cement . id wet the cement circle first or better still paint it with pva and water too give the bricks a good bond . use 3 parts sharp sand too one part Portland cement . don't make it too runny .

How high will your brick circle be? Premixed dry mortar in bags is availble for laying bricks, the key is the word MORTAR! There is a difference between mortar and cement. Mortar is the paste used in binding bricks or stone together. Sharp sand cement is too dense and will not enter into the pores of the brick well enough for locking and adhesion. By the way Noseypotter potter has given you a good ratio for brick laying cement if you care to mix your own and not bag it. The mistake would be to lay bricks using a cement ratio used for forming.

Not sharp sand for making mortar, you need Builders soft sand for that. And you can add some of the PVA adhesive to the water when you make the mix, helps it stick to the bricks.

No need to use cement or mortar just use sharp sand. If you don't get the bricks quite level or lined up, sand is a lot easier to deal with than mortar. depending on how tightly you butt the bricks together, brush kiln-dried sand or horticultural grit over to fill the gaps when you've finished. Or leave bigger spaces and fill the gaps with fine growing material and plant low growing aromatic plants.

I built a brick circle for a friend last year. I wonder what diameter yours will be and how many courses of bricks you'll be laying ? And whether your wall will be 9 inches thick ?
As Nosey says you must find the exact centre then hammer in a steel bar firmily, verticularly, so it's top is a little taller than your finished wall height.
Then get a length of timber - say 2 ins x 1/2 ins, a little longer than the radius of your circle.
Cut a point at 1 end of it and drill a hole through the other end to fit nicely over the steel bar, so that the distance between the pointed end to the centre of the hole is equal to the desired radius.
I can go on if necessary. All the best.

You said you've dug out a circle and laid the foundations for it and your friend has already started it for you? So you probably already have the dimensions and the radius sorted. I would just carry on on from where your friend left off.

Regarding what sand and the ratio of sand to cement to use. I would use the same ratios and type of sand as was used to build the first bit. If you don't then the colour and texture of the mortar won't match and it will look odd.

A tip that a bricklayer gave me was when mixing the mortar add a squirt of washing-up liquid to it. This makes it more pliable and easier to work with.