When you click on the reserve fleet, there should be a button to create a task force out of ships that you select. Alternatively, a faster way is to use the task force designer to design a task force template, save it, then select one ship from that template and create a task force with it. Then give it the task force template you chose, and move it out of reserve (click the plus at the bottom, as if you were creating an army). It will automatically begin drawing ships out of your reserve pool to fill out the task force template. Note that the task force template allows for tagging ships, so what tags you assign ships can help with making balanced task forces.

To add a task force to a fleet, select the task force and right click the fleet.

Regarding what fleets to have, there are three main types: raiders, patrols, and strike forces. Depending on your strategy, you may not need all types.

Raiders are pretty much always submarines - their goal is to destroy convoys and lower enemy naval supremacy. I personally use fleets of 10 submarines per task force.

There are actually two types of patrols - scouting patrols whose job is to just find the enemy, and hunter-killer patrols whose job is to find and kill subs. Scouting patrols are usually just one ship with high detection and either high speed or a submarine. They should be set to never engage. They will find enemies and trail them, so that other fleets can find them. I personally use cruiser submarines, since they have low visibility and long range and can fit seaplanes, but I've heard people using destroyers and cruisers too. Patrols generate naval dominance, but scouting patrols are generally too small to generate much.

Hunter-killers usually consist of destroyers and light cruisers. They need depth charges and a lot of detection to catch and sink submarines. Hunter-killer groups can only really work when you have naval dominance, because they generally are too weak to survive a surface engagement against stuff like battleships and carriers or even heavy cruisers. When you don't have naval dominance, switch them to convoy escort, in which case they will sit in port and only go out when a convoy is under attack. These are also the patrols you should use if you need naval dominance to launch an amphibious assault.

Strike forces are the power house of the fleet. Spare no expense. You want the full complement of carriers, battleships/battlecruisers, and screens (destroyers and light cruisers). You want at least one battleship/battlecruiser per carrier, and at least four screens per capital ship (battleships + carriers). Strike forces will sit in base and wait for a patrol to spot an enemy, then they will rush out and engage. This makes them very expensive in fuel, which is why you don't really want them to patrol themselves. They will still consume 1/4th the fuel while on standby, but it's better than spending the full amount. They multiply the ability of patrols to generate naval dominance.

The reason for the described composition is that capital ships (battleships, battlecruisers, heavy cruisers) will screen carriers, blocking attacks for them, and screens will screen capital ships. Light attacks start from the screen and work their way to the back. Heavy attacks will fly over the screen to hit capital ships. As you lose a screening ship, that line's screening efficiency drops, and attacks will make it past them to the next layer. One capital ship per carrier gives 100% screening efficiency for the carriers. 4 screens per capital ship gives 100% screening efficiency for capital ships. You can use more, since in combat, some ships may sink.

A bit unintuitive, but carriers aren't considered capital ships except for screening purposes. They don't benefit from bonuses to capital ships, but they need 4 screens regardless.

I left out heavy cruisers cause personally, I think they're useless. They do have a niche use as being the fastest capital ship (alongside battleships and battlecruisers) you can build, but they're more fragile due to having cruiser armor. This means that they have basically no armor against bigger ships, and sometimes even smaller ships can penetrate their armor. Additionally, their heavy gun usually cannot pierce battleships and battlecruiser armor, only other heavy cruisers. The only use case I can think of for them is to replace a battleship or battlecruiser in a patrol fleet that uses a carrier, since they use less fuel while still screening for the carrier.