Just be honest. If you think you have done a great job at something, don't be afraid to say so. One thing my managers have asked me to do is make some reference in the self review to every major project I had worked on since the last review. Sometimes (especially if it's a year between performance reviews) managers can lose track of all the great stuff their employees have done, so it's good to remind them once in a while.

On the other hand, if you know there are some things your boss will probably put down as things you need to improve, be proactive about acknowledging them and explaining what you plan to do to make them better. That makes you seem more responsible about addressing any shortcomings than simply ignoring them and having it look like you had no idea when your manager writes them down on his/her form.

If the fact that you just started means you don't have much success or failure to report at all, it's fine to write that down too. Your boss is aware that you started recently.

Ideally nothing on your performance review should be a surprise. If you're not getting informal feedback from time to time (positive or negative), you should ask your manager for some more of it.