Stellaris

Note: Due to ongoing changes to the game over time, some tips can become outdated. The current advice is accurate as of version 2.5, and a maintained beginner's guide can be found on the official wiki.

Military & Warfare

Different weapon types are weaker or stronger against different types of defenses, so when using manual ship design and considering how to gear up your ships, it's best to focus on your opponents' weaknesses. For example kinetic weapons are strong against shields but weak against armor, so if you take a look at your neighbor's ships and see they're heavy on armor but low on shields, you probably want to avoid going hard on kinetic weapons for your own ships.

Use Starbases to block strategic chokepoints and to guard your borders. Border Starbases should always be fitted with extra firepower and preferably a Listening Post which allows you to track the other nation's nearby fleet movements.

Don't forget to make claims on enemy systems before declaring war on them. Claim costs increase the further the claims are from you.

Even basic Outposts can have some Defense Platforms built around them, handy if some small-scale nuisances like pirates keep up gunning for the same system. They are quite expensive and fragile without fleet support however.

Specialization of planets is important. Colonies can be designated to be more efficient at producing a certain type of resource and should focus on producing that resource - for example a Tech World should be filled with Researchers, a Forge World with Alloy production, and so forth. Colony designation is automatic by default, but it's best to choose them manually.

Empire Sprawl increases with the size of your empire and will impose a penalty on costs if it exceeds Administrative Capacity. Have at least one colony dedicated to Bureaucrats who increase Administrative Capacity, especially if you plan on expanding rapidly.

Prioritize grabbing systems with large colonizable planets, systems with high resource quantities, and systems that make good hyperlane chokepoints. High-quality but low-habitability planets can become useful later, either via terraforming or populating them with robots/more suitable species.

Food is 100% an empire-wide resource, meaning a planet can generate negative amounts of food and still grow normally if your empire is otherwise producing excess food. Excess food has no use beyond selling it in the Galactic Market, so it tends to be the least important of the basic resources.

Energy production is important, a sizable late game fleet can put you in a pretty big deficit if you don't keep up with it. Keep your fleets docked in a Starbase with Crew Quarters built when they're not in use to reduce their maintenance.

The general agenda of other empires can be deduced by their trait description. "Spiritualist Seekers" will like other spiritual empires, and "Fanatical Purifiers" will probably try to kill you at some point. Gifts and deliberately favorable trade deals are usually the only way at first to improve relations, until you can get a treaty which starts building trust.

Look through the Ascension Perks tied to your traditions at the start of the game, and try to pick a few to go for. Some are locked behind tech, so it's helpful to decide which type of endgame you want to pursue so that you can pick the techs as they come up.