LearnGaelic is a great source for beginners because it provides a simple overview of the Scottish alphabet, the sounds each makes, and so on. It is a great accompaniment to other language learning sources. It is funded by the Scottish government so it also goes through rigorous review.
An Comunn Gàidhealach Ameireaganach (The American Scottish Gaelic Society) provides a list of in-person sources for learning to speak Gaelic, as well as contact information for instructors and clubs.
The Gaelic College is a premier source for learning to speak Gaelic at varying levels. Its Atlantic Gaelic Academy offers a variety of courses and online video lessons that will have you speaking fluently in no time. They also have programs for youth. The courses are not free, but the online video lessons are.
Sabhal Mòr Ostaig is a Gaelic college on the Isle of Skye that offers a year-long online program called An Cùrsa Inntrigidh for people interested in learning Gaelic. The program’s learning materials are online, but requires students to participate in tutorials once per week via telephone. Once you’ve completed the course, you can take university-level Gaelic courses in person or online.
The Dictionary of the Scots Language is just that…a dictionary of Scottish words as well as translations of phrases. It includes modern Scots (1700 onward) and Older Scots (12th Century to 1700).