A semicolon joins two complete and related ideas.

"I need to buy some things; I'm having a party."

A colon CAN join two complete ideas. The first idea must be complete, and the second should be a list, or a definition or explanation of the first idea. Sometimes it's used to introduce a name.

"I need to buy some things: napkins, drinks, and ice."

"I'm having a party: a gathering of friends, with dancing and snacks."

"You might know one of my guests: Shakira."