What do you mean? Are you asking, "How do I increase my fleet supply?" or are you asking "How do I increase the maximum fleet supply above the number you would get with the Large fleet supply setting?"
If you're brand new to the game and wondering how to increase your fleet supply, the answer is to find the Fleet technology tree where you will see two progressions of 8 research steps each. The one on the bottom is for your fleet supply; the first increases are cheap. Do note that when you increase your fleet supply you increase the amount of credits, metal, and crystal lost to taxes.