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.