Something they know would make the deceased proud to hear of. Grieving for the death of a loved one is a long process that takes time and acceptance.
Even so, it does not have to be looked upon as a daunting task. Personal experiences are expected to be re told and reminisced, and to be shared with other people who will be listening. You need to take this time to share your loss, to laugh and talk and share your thoughts and feelings with each other.
It is wise to develop a theme to give focus on your speech. If the deceased was a practical joker or light-hearted individual, maybe a poem that incorporates a bit of humor would remind their family of what a happy spirit that individual was. If more than one person is delivering a eulogy, coordinate with them so both approaches are used.
It is important that you avoid any statements or references that portray the deceased in a negative manner. As hard as it may seem right now, it gets better.