Hello Sandra,
I think you have to be careful of who you choose to do your website. Some SEO companies claim to guarantee you a number one position in top search engines etc, but these number one positions may not necessarily be on your most important website keywords. Getting a number one position or top ten position for that matter on keywords that are rarely used to search for your products or services are not going to give you a great deal of extra traffic.
Also be careful of SEO's that "Spam" search engines by creating many doorway pages leading to your website. Once a search engine realises you're using doorway pages you're likely to receive a URL ban in that search engine.
So if you're going to use an SEO it's best to ask them exactly how they are going to do it.
Additionally - don't let them change your website URL's unnecessarily as all these pages that are already indexed will be lost and send your potential customers to 404 error pages - web page not found.
So while SEO can be helpful there is definitely a right and wrong way of going about it.
I have an interesting article on SEO on my website written by David Wallace of SearchRank, called Spamming Techniques That You Will Want To Avoid! . This article covers some of the things that shouldn't be done.
Additionally there are quite a few other articles written on SEO.
Hope this helps.