Almost every patent application gets a letter just like that. Your next step could be to find a patent attorney or patent agent to help you through the process. If, instead, you plan to continue on your own, there is a lot to learn.
A document called the MPEP is the "bible" for examiners and applicants in terms of procedures. It has a section on the manner of making amendments. That covers the mechanics of replying. But first you need to completely understand the Office Action and formulate a plan to get past each specific objection and rejection. If the examiner is correct on some issue you will need to amend, if you are confident she has made an error, you can argue the point rather than amend. In doing that you can't add new matter. Amending claims to cover something explained well in the initial filing is fine.
At this stage you are entitled to an interview with the examiner and her supervisor, usually by phone. It lasts 1/2 hour and is to give you a chance to understand how she sees the cited prior art as the same as what you are claiming and for you to clarify misunderstandings she may have about your invention. You can talk about your proposed amendments without putting them on record and get her reaction.
You draft and submit your response to the office action after getting the benefit of this conversation. This is an unforgiving and arcane process. Please get, and study, Patent it Yourself by David Pressman. Also you can look up the history of other patent cases in your field via USPTO Public PAIR to see how responses where formulated. One small tip - if you submit an amendment that is not signed it has about the same effect as not having sent it in at all.