Wikipedia is designed to make editing very approachable for new users. When you create your account, Wikipedia will automatically offer to take you to a suggested article for editing.
If you do decide to edit the article that is suggested for you, this will launch a guided tour of the editing process, which can make the process easier if you aren't sure how you can contribute. This process also emphasizes how you can become a Wikipedia editor without needing extensive background knowledge in any particular field.
Do you prefer to learn through games? Check out the Wikipedia Adventure to learn to edit while you play!