You need to make more colonies (and then resource facilities on them) to pay for your ships.
You can take a bonus in the Maintenance Reduction trait when designing your empire, so you pay less maintenance. But be warned: it reduces the base maintenance, not the maintenance paid. The base maintenance cost is 25% of a ship's cost per turn. The Maintenance Reduction trait reduces this base cost. So, 110 in the trait means you pay 15% maintenance per turn for each ship you have. Bases cost maintenance too. Units do not.
You can mothball ships, and then you pay no maintenance on them. If you do that, they can't do anything at all. You can unmothball them later, and that costs some money. You have to have the ships in the same sector (square on the system map) to mothball and unmothball ships. Select a ship, and hit the g key. There are other useful things to do in that window besides mothballing.