Normally, ships built on the first turn of a campaign enter the game immediately, whilst those built on subsequent turns take a realistic time to construct.
Sometimes - such as when you want to model historically accurate scenarios - you may want to delay the entry of ships that are ordered on turn 1.
You can do this when you are giving orders to build the ship.
To delay the launch of any ship ordered on turn 1, on the build ships screen, set the "weeks to commissioning value" to the desired figure, and click on the "Set" button:
Make sure you do this before you click on the "Build" button.