Visit a local bookstore (large-scale or specific college or textbook store) or search online book shops to purchase textbooks and workbooks. One of the most highly used Scottish language textbooks is the "Hugo Language Course: Scottish Gaelic in Three Months" by DK Publishing.
Consider investing in a comprehensive computer program such as Rosetta Stone. Other technologies, such as podcasts (Apple, Ipod) or audio lessons greatly increase comprehension as opposed to only studying the text. A mixture of both reading and listening to the words is the most effective way to learn a language.
Supplement your studies with actual Scottish Gaelic language texts or music. It is much easier to learn if there is enjoyment or interest in the method. As you are on a self-course, choose texts based on subjects that are most interesting to you. Children's books, films, music or browsing classic Scottish texts enliven and extend study. They may be more difficult to find, but with resources like ebay and Amazon.com and libraries, finding such items make the process a little easier.
Join chat rooms, email exchange and online voice chat. Get connected and find other people who are learning or who already speak Scottish Gaelic to both practice what you have learned. Chat rooms and message boards are great places to have doubts remedied and questions answered. A popular language exchange website is My Language Exchange (mylangaugeexchange.com), where you can connect with speakers of just about any language, including Scottish Gaelic.