There are two things essential to making ethical judgements about one's own behaviour, first having criteria for making such judgements and second ability to be honest with oneself.
For criteria, one cannot judge whether an action is good or bad without first having thought through what constitutes goodness and badness. The weaker one's judgements, and the less they are the result of extended and serious reflection, the more likely that you will just change you criteria in order to feel good about doing whatever you wish to do anyway. Thus a fairly solid code of morality is important.
The next thing you need is the ability to be honest with yourself and clear-sighted. In other words, if you think stealing is wrong, and you find yourself saying "but Walmart is a big company and they won't miss it" or "but having five new shades of nail polish will make me popular at school and that's really important", you are deluding yourself to excuse your actions.
You might consider asking the three questions Kant recommends:
What would happen if everyone acted as I did?
Am I treating all intelligent beings (human and animal) as ends in themselves or just means to my own ends?
Am I acting out of moral motives?