Preparation = Confidence
Being prepared will give you confidence to know what to say. Imagine this is an interview; you need to positively communicate why you are an asset to the firm. Look back at the last year (use notes, calendar, a month at a time) and list out all your accomplishments. Your manager may not know all the things that you have done. It is your job to communicate them.
Review and summarise achievements
After you have an initial long list of all your achievements, review and determine which ones are the most relevant and the ones that you are the most proud of. Summarise the edited, shorter list of "key accomplishments".
Provide / reference independent evidence
Evidence your accomplishments with independent statistics if you have them, or emails from clients/customers thanking you for your work or the great job you did. Make sure that you add names of clients/customers with their job titles. Praise from a junior team member or a senior Managing Director all hold their own merits.
Relate them back to your goals
You should refer everything back to the goals set at the start of the year. Where a goal has not been fully met, evidence why and pro-actively suggest remediation if appropriate.
Communicate your career aspirations
Make sure that you know what is expected of you in your current role. Then communicate what your career aspirations are in the short and long term. What do you want to do? Why should you be considered for the next career step?
Set and manage expectations
Use the review to set expectations. Don't see it as a one-off meeting, however. It is your responsibility to manage your career and give your boss updates on how your agreed goals are going. This is an ongoing process that should occur regularly, say every 2 months from your review, until the year end and should always be evidence based.