How do I give performance review?

Am a developer, this is the first time i have been asked to give a performance review for employees working with me. So, in this process I don't want to lose the relationship with the employees (with a honest feedback) and at the same time I shouldn't demotivate them with a lesser bonus. On what all aspects can I be evaluating an employee ? And what should i not even 'touch' ? PS: I am nervous because the review I give is going to impact (both positive and negative) his job, in turn his career. I just don't want to make a blunder and excuse myself saying this was just a first time.