Stellaris

Note: The following advice is accurate as of 2.1.

Military & Warfare

• Lasers beat armor, kinetic weapons beat shields, missiles beat a little of both and are more accurate, but can be shot down by point defense. Normal guns and lasers are not point defense.

• 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.

Expanding & Planet Management

• Grabbing territory will increase tech and tradition costs, planets moreso than systems. Small and poor planets may not be worth colonizing if their output isn't worth it.

• 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.

• Pirates pop up in unclaimed systems adjacent to your claimed systems, an incentive to eventually fill in any unclaimed bubbles left over in your territory.

• Food is 100% an empire-wide resource, meaning a planet can generate negative amounts of food and still grow fast if your empire is otherwise producing excess food. As time passes and your planets are filled, excess food production can be replaced with something more useful.

• When you start making Sectors, look at their planets and see if they lend themselves to a specialization rather than just generic resources. They may have really good energy planets for instance.

General Advice

• 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.