I've been thinking about... - This is a useful phrase to show you are considering something.
That's exactly right. - We use this phrase to confirm something
I signed up for... - When you sign up for something, you register to do it.
Course (n) - A course is a collection of lessons or a class that teaches a set of lessons about a topic or subject
Certificate (n) - A certificate is a document that shows completion or mastery of a subject or course
Field (n) - A field refers to a subject or area of study in the working world