Learn the Lingala alphabet. Luckily, Lingala uses the Latin alphabet. There are four tones in Lingala, however, so you will have to learn the tonal marks, as they are sometimes included in writing. The Lingala alphabet is easy to learn for English speakers.
The Omniglot website has a resource page dedicated to the Lingala alphabet, separated into vowels and consonants, with an explanation of the four tones. But there is one thing which will take some getting used to: There is no standardized spelling system. Lingala is used primarily for speaking, and the literacy rate among people who speak Lingala is fairly low.
Start building vocabulary. You can find an online Lingala-English dictionary at Freelang.Net. You can download the dictionary straight to your computer if you are running Windows. You can begin learning and putting together phrases as well. There are many helpful conversational phrases listed on the Mashada Online Community Board. There is also a list of Lingala Phrases featured in the Resources section of this article. Here you can learn basics like "yes," "no," "please" and "thank you." But there is also some vocabulary, such as how to say "coffee" and "tea," as well as a couple of expletives not suitable for children.
Listen to Lingala. No matter how well you think you are pronouncing the words based on written instructions, you never truly know until you hear a native speaker speak. The Dictyon Lingala Materials page has many audio files for you to listen to the sound of spoken Lingala. There are also many songs available in Lingala. You can find several Lingala songs for free at the Lingala Songs Playlist page listed in the Resources section of this article.
Practice. Write as much as you can in Lingala. If you live near someone who actually speaks it, speak as often as you can in the language. If you have learned a good amount of vocabulary, you can try your hand at one of the crossword puzzles at the Dictyon Lingala Materials webpage. The crosswords can be rather challenging, but puzzles do have a reveal option, which shows you either the first letter of the word in question or the entire word. These crossword puzzles are also good ways to learn vocabulary.