Some things to mention at a memorial service include the person's background and accomplishments, your personal relationship with the person and how he affected your life, and the legacy the person leaves behind. Some other ideas include discussing anything that made the person special or unique, and funny or sweet memories and anecdotes.
When speaking of the person's background, discuss how historic events or personal challenges shaped his character. Talk about the person's milestones or special accomplishments, and what it took for him to become the person he was. If there is something the person left behind as a legacy, discuss what it is, whether it is a quality he instilled in others or a big achievement that changed the community or world around him.
Discuss how you met the person or were related to him, as well as the type of relationship you had with him. If there was a special thing the two of you did regularly, discuss this and how it affected you as a person or what you learned as a result. Another option is to talk about a cause that was special to his heart, such as work with a homeless shelter or children's mentorship program.
It is appropriate to use humor in the service, as long as it is positive and complimentary. This includes happy memories or anecdotes, such as a funny moment from a family gathering. Avoid any topic that makes fun of the person or highlights a shortcoming he had.