1. Click on the Settings button at the top
2. Click on Domain Settings at the left, followed by Spam Filtering
3. Click Overide Spam Settings for this Account
4. Click on Actions and then set your required % weightings

Refer here for the meaning of each criterion. Note that only the Admin login can set the percentage weightings. You can also use Content Filtering to filter keywords in selected fields.

In addition you can use whitelisting (Trusted Senders) to make sure emails from your trusted contacts get through.