Visual design for websites can start with a simple sketch drawing, or roughly proportioned blocks in Photoshop, to gain a principal layout strategy.
Designers have different strategies on successfully designing a page. Mine starts out with a rough grid and container diagram.
The content will lean to a certain style of layout.
I tend to start with a very simple 12 column grid, using the extremely popular960 grid system. This can be on paper or in Photoshop. Then after studying all the planning documents, a simple shape-based layout is formed. Containers for the main navigational elements and content wells are roughly appropriated, along with ideas of navigation styles and other functionality requirements as laid out in the functional specification, I.A. and brief.
Web Design Ledger have a great post collating all the best grid tools and resources here
I tend to design pages using layers of finesse, from basic to highly polished. It is easy to lay on too much aesthetic from the outset which can make attaining a good layout much harder. Keep adding layers of finesse and always add slowly, until a good solid visual style is apparent. Depending on your relationship with the client, you may wish to show your process to them as and when, this minimises any major issues, and means that you wont throw too much away.
Colour Theory is important at this juncture, taking into account legacy, branding and accessibility. Again the hero colour should be chosen and a good secondary palette using one or two complementary colours. Adobe Kuler is great for this, giving you options such as complementary and analogous.