After being promoted to Commander and joining the Enterprise, Riker became First Officer and immediately put his command skills to use. He usually leads away teams that beam down to planets on dangerous missions, and he often works alongside Beverly Crusher, Data and Jordi La Forge on missions like that.
He's also had to take command of the Enterprise on many occasions if the Captain is sick, injured, away or (in some cases) kidnapped. Riker respects his Captain more than anyone and he actually refuses to be given his own command, preferring instead to stay on the Enterprise. After many long years serving there, he was eventually given command of the ship when Picard was promoted to Admiral.