It is not easy to answer this.
a. Has this been a long time issue in learning and work? If so, I would consult a doctor first to isolate medical causes.
b. For "normals", remove distractions: TV, music, telephone, text messages.
c. form habits of breaking work in to bits, and getting them done, a very brief break (NO TV), then next task and so on.
How do I work efficiently without getting distracted?
2 answers
Bobpursley's suggestions are all great. You might also promise yourself a small reward after you finish a specific task.