The first thing you want to do is to get permission. You mentioned that you have a database of email addresses, but you don't mention whether or not you have permission and by the wording of your question, my guess is, you don't. There is a piece of legislation with which you'll want to familiarize yourself, it's called the CAN-SPAM Act.
Once you have educated yourself about the legalities of email marketing, it's time to find a reputable emailing list service. The ones I prefer are:
* Aweber - their software has a bit of a learning curve, but a healthy and very helpful how-to video library helps and you can do all sorts of things like split tests and get all sorts of metrics and reports.
* The Right Web Tools - they offer a modular service that only gives you the tools you need when you need them so you're not distracted by extra bells and whistles.
* Constant Contact - If you just want to be able to send newsletters, this is the ticket for you. It doesn't come with any autoresponder features, but they offer great newsletter templates and you don't need to know any fancy coding.
* 1Shoppingcart.com - If you're also going to be selling products, you might want to use the list management features that are integrated with the third-party shopping cart services offered by this company.
With most of the above systems, the emailing list service provider will ask you to make sure everyone on your list opts-in. This means, you'll be allowed to send them one or two notices asking them to click a link that will give you permission to market to them. Keep in mind that many marketers sometimes lose 50 percent of their marketing list during the opt-in process, but that's okay. You don't want anyone to be on your list that doesn't want to be there.
Plus, with a full opt-in list, open rates and product sales done through email are more likely because when people opt-in, it shows they're really interested in what you have to say.
The last steps are to make sure you offer top-notch content and figure out a way to be consistent. No one wants to get a newsletter that doesn't offer any helpful content. Also, the more consistent you are, the more people begin to look forward to receiving communications from you. Use your calendar to stay on top of your marketing - depending on your audience, once per week or twice a month is a good schedule.