This will usually be done using software like Photo Shop and other professional design packages which will enable much higher resolutions, much more precise measurements and generally the most attractive outcomes.
However you might also decide that you want to include some photography in your web design – maybe of happy customers or of your staff. You can use actors here or pose for photos yourself and have these then retouched afterward. If you are making the images yourself to use then it is highly advisable that you use a website design company to handle this part, outsource the images using a webmaster forum, or at least make sure to invest in the right software and to spend time learning how best to use it.
Next you are going to actually have to code the site, or your website design team are at least. This is where your site stops being an idea and begins to form into something tangible and real.
There are various types of programming languages used to define the layout and the behaviour of a website.