To improve your TOEFL score you will want to focus on both test-specific and general skills. These blog posts provide suggestions for improvement in each area of the test:
Essentially, you want to use both resources that help you understand test-specific structures and sources that help you use language skills in general. This blog post can lead you to some of the best free resources.
Start by going through the Magoosh lesson videos and practice questions. At the same time, find ways to use English as much as possible. You could have a conversation with a friend, listen to a podcast or lecture, watch a documentary, or read an article. As you listen and read, practice writing by summarizing the lecture or article.
In addition, you'll want to improve your vocabulary. The best way to do so is to read — that way you'll learn vocabulary in context. Make flashcards of words you don't know, and note the context in which it was used. We also have Magoosh TOEFL flashcards for basic academic vocabulary :)
Our blog has many more ideas on how you can improve your studies. Whenever you feel stuck on a particular section, it's a great place to look for new ideas about how to prepare.