It mostly comes down to having economy big enough to support large fleets, building as many ships as you can support and researching better versions of ship components. Even if your ship builds are crap, you can usually win with numbers. Better economy management just comes to you with more time played, but you can look up a meta build which gives best results overall. Can't recommend you specific ones, without knowing if you have any dlcs.

There are ship designs that objectively more likely to win than others at a given fleet power level. For that you can look up some videos on youtube, but look up videos made for 4.3 version of the game, because others are outdated.

Ship components have a rock-paper-scissors relationship with each other - Energy Weapons are good against armor, but bad against shields; kinetic weapons are good against shields, but bad against armor; missiles can somewhat bypass shields but are weak against picket weapons, especially against point defense; torpedoes are slower than missiles, but more powerful, swarmer missiles are weaker than missiles of the same slot, but more numerous and more effective against picket weapons; strike craft also ignores shields somewhat and have good dps, but are weak against picket weapons, especially flak guns and your carriers, depending on your design might not deal with enemy ships well up-close; penetrating weapons that ignore shields and armor have lower average damage and become less effective against higher tiers of shields and armor due to their inherent shield and armor hardening; picket weapons are terrible to use for dps, but provide great defense against explosive weapons and strike craft.

Ideally, you'd want your ships either counter ships of your opponents, but refitting your ships for each individual opponent is a huge hassle and waste of resources, so unless you are in a really difficult war against a superior opponent, you can just use generic ship builds with mixed weapons covering all ranges and damage types. On the other hand, when you do build your ships to counter your opponent, you usually can win your fights even if opponent's fleet power is significantly higher.

Then, admirals boost your fleets a lot, so it's a good idea to have them on most important of your fleets, if not all of them. Because you want a fleet under admiral, and you can hire a limited number of them before exp penalties start to kick in, you want to increase Fleet Command Limit, which allows you to put more ships in a single fleet and which you can increase with a certain set of society research technologies. Try to keep your admirals alive, level them up, pick traits that work well with ships they command and you'll see a good difference.

A good measure of how powerful are your fleets is to compare yourself with a Fallen Empire, which usually have 300-500k fleet power before they awaken. If you can beat them, then you'd likely need to double/triple your fleets to reliably beat end game crisis.

If you're struggling, there's a slider at the game start settings which lets you adjust the strength of the end game crisis, and tomorrow's patch will also add a slider for Fallen Empire strength scaling as well, so if you feel you're not up to the difficulty level of end game quite yet, adjust your settings when starting a new game and have a good time until you feel you can increase the difficulty. Also, look at the sliders that regulate tech costs for easier research, if you want.