Sir, I have built three such ponds in my small garden, two for fish and one as a small habitat for frogs which soon found it and they are great for helping to keep snails and slugs down.
The size and shape of the pond depends upon the size of your garden and how many fish you wish to keep. I used cheap, re-claimed bricks. Obviously, you need a level surface. Then lay the bricks out 'roughly' in tiers to see what it looks like and dismantle them when you know what you want in terms of shape. Buy the ingredients for mortar. Experiment with a couple of bricks to see how you can achieve the right consistency of the mix. You can also buy plastic 'formers' to ensure that you get even layers of bricks - and a trowel, of course. I mixed my mortar in an old wheelbarrow. If you are not very good at bricklaying, seek further advice. My ponds are not expert bricklaying in terms of evenness but they don't have to be perfect. In fact, they provide a certain 'rustic charm', especially as trailing plants run over the sides!
Go to a garden centre and buy some of that heavy-duty plastic liner, large enough for the pond. Trim it to size, leaving an overlap at the top tier of bricks, so that water does not leak out. Cut patio slabs on top are great. For a small pond, you can even buy one of those ready-made plastic moulds and build the brickwork around it. Gaps can be filled with soil for planting. My pond looks fantastic when grasses, flowers and marginal plants come up. I did this for one of my ponds and it has lasted for years and is still fine. Do not build a pond up against your house wall as this could cause all sorts of problems with damp and mould.
Buy some gravel (large size at about one centimetre), wash it thoroughly and place a layer of a few centimetres at the bottom of the pond. I used cheap, white gravel, which soon attracts algae and 'colours down'. Makes a good medium for plants such as water lilies - good shelter for fish, too.
You will need a pump powerful enough for the pond size, for filtration and aeration. These can be expensive. Ensure that you have the means to 'trail' the wire, say, around the garden edge, through to an inside mains plug. I fitted my pumps with cheap garden spray devices to achieve great fountains, which can be varied according to the setting you choose. Finally, I placed cheap garden lights in 'strategic positions' for wonderful effects at night. The 20watt bulbs have been going for ten years in all weathers!
When you fill the pond with water, leave it to settle for a few days before introducing fish. Do not buy expensive specialised fish such as koi carp, as they need greater care and are more sensitive to water conditions than 'ordinary' goldfish and shubumkins. These are hardy, even through the winter. I have had mine for years!
These are my 'poor man's' descriptions for sunken ponds. My neighbour bulit a large raised pond, laid on patio slabs. It is rectangular, for ease of building, but being raised, had to have much more careful brickwork, especially as it is designed for people to sit on it (cut patio slabs on the top surrounds).
Some tips about maintenance. You will soon get masses of pond weed. Be prepared to remove this from time to time before it chokes the pond. You can buy slow-growing aeration plants and lilies quite cheaply.
Have the pond in an accessible position where you can reach it for cleaning and changing water and for taking out the pump and cleaning out its sponge and other components. (Switch off the power!) There is nothing worse than breaking your back to do this! You will see when the water needs partial changing. I still use buckets and a garden hose for my small ponds - but it only has to be done every month or so in the growing season.
In winter, the pump(s) may freeze up and you may have to swtich them off for a time. No problem: the fish will survive under a frozen surface - but break it up if it does freeze. You may need some mesh to cover the pond against cats, birds, etc. This is practical if the pond is only a few feet wide. I bought some of that metal grid-shaped mesh which I put over the pond at night. When the pond is mature, fish should have plenty of room to hide!
Sorry for the long answer - but I thought that you might need to know other things, apart from the brickwork. Weigh up the costs of materials and equipment to see how much you wish to pay - or if you can afford it. I 'begged, borrowed and stole'. If you can do this, so much the better. It's a hassle to start with but well worth the results, as for any garden venture. Best wishes.