First, you need to determine - and as precisely as possible - the scope and content of your site. By doing that you'll have a good idea of how many pages it's going run to and how much your site's going to cost. Are you offering a product or a service which can be bought directly off your website, or do you want leads? If you're selling a wide range of products, your website will need a 'shopping basket' plus a facility for taking credit card payments. Sales-based websites do, generally speaking, take longer to develop than lead-generation websites and are therefore more costly. IMPORTANT: skim over this aspect of the process and you'll be making changes later on, which will cost you money.
Knowing what the content of your website is going to be, you then need to decide how it's going to 'work'. What's the natural progression of the site? How should it be divided up? Which pages link to other pages? The better the site is planned, the easier it will be for visitors to find their way around it. You should also consider at this stage how frequently your site is going to need updating. Would you prefer that to be done by the people who are going to create your site, or do you want to do it yourself? If the latter, then let your web developers know before they begin work. IMPORTANT: sound planning can also help you identify any content that you may have overlooked, or forgotten to include, in stage 1.
With the quantifying and planning dealt with, then comes the writing and design of your site. The writer and/or the designer will expect you to provide (in written form) comprehensive information about your products or service and any images you want including on the site. Before you brief your designer or writer, find some sites you admire: designers in particular find it helpful to know what you like - and dislike - before they start work. And what about the words - i.e., the 'copy'? Are you sufficiently skilled to write the site's content? Good copy contributes to a site's success: amateur copy, sloppy punctuation and grammatical mistakes on the other hand will destroy the site's credibility. IMPORTANT: be honest with yourself - do you have what it takes to write easy-to-read and persuasive copy?
If you want as many people as possible to find your website, then it should be 'optimised' at the same time as it's being written and designed. Although not essential, optimising your site does involve extra costs, but it can pay dividends by driving more traffic (visitors) to your site and reducing the need to spend money on advertising - conventional or otherwise. Try to make the decision to optimise or not ASAP as it's easier to optimise a site as it's being developed rather than later. IMPORTANT: the name of your website (the 'domain name') can play a big part, in getting your site found by the search engines, so when you get round to it, choose the domain name carefully.
Once all the quantifying, planning and creative issues have been dealt with, it's time to start the actual building of the website. Although this is the 'technical' side of the package, the 'build' can (depending on the nature of the site) be a relatively quick and inexpensive part of the process with no requirement for you to be involved at all. Once built - and before it goes into the public domain - you'll be able to put your site through its paces to check that it's working like it should. IMPORTANT: making substantial changes to either the content or the creative at this stage, may add significantly to your costs.
You've completed your testing and it's time to publish and make it available in the public domain. For anyone to find your website, it has to sit on a computer (a 'server') - 24 hours a day, 365 days of the year. There are businesses all over the world which provide that service ('hosting') although it's likely to be in your best interests to use a hosting provider that's based in your country. And once your site is published, don't expect instant results. It may take anything between one month and six months for it to be 'found' by the major search engines. IMPORTANT: always pay your hosting provider on time, otherwise you may not have a website any more...