Website design firms come in all shapes and sizes.
When choosing which firm you would like to build your website, there are many factors which you need to take into consideration.
There is no point having really nice looking website if no one can see it because it has poor optimisation.
Most web desgn firms will offer additional services such as optimisation, website promotion and may also submit your website to the search engines.
For me the website they have created for you should be fully optimised anyway and I would also expect the web design firm should submit the site to the search engines as a matter of course. It is always worth asking to make sure.
Once they have produced what hopefully is an outstanding website you then need to ask them if they would include a link to your homepage from their own website, preferably their homepage. Search engines these days take these backward links into consideration when they decide where to position your site in their results. As a general rule of thumb the more backward links you have the more visitors to your site you atre likely to have.
If you want to promote the website even further, google adwords is a favourite of mine. This is where you pay to be seen on the right hand side of the results in Google, in the sponsored search section.