Well, for something like this I use http://www.ceantar.org/Dicts/index.html
at Ceantar.org. It's a collection of dictionaries for different dialects of Gaelic; they're not entirely comprehensive and are certainly not what you would want to learn any form of Gaelic, but they're fine for looking up individual words. The format is a bit outdated but perfectly serviceable.
They also have a list of Gaelic lessons online; the first thing to do of course is to decide what dialect you'd like to learn. Probably, you'd do Irish Gaelic as it's the most widely spoken (which isn't saying THAT much, but it's something) and there are the most resources available for it. That list is here: http://www.ceantar.org/Lessons/index.html
However, if you're serious I would suggest checking out Teach Yourself Gaelic (there's a book and accompanying tape/CD). Your local library may have it, or you can get it from Amazon.com (although if you're going to buy it I have to tell you to first check a locally owned independent bookstore and see if they can order it for you! Much better to support local businesses!). The Teach Yourself books are a well-respected language learning series, and I've found them very helpful and nicely done.