Pokémon Ultra Sun and Ultra Moon

General

• Use whoever you want to use, you don't really need to feel obligated to use a 'competitive' team or whatever for an in-game playthrough. Some are better than others, of course, but the game doesn't really penalize you for using the "wrong" ones.

• Don't be ashamed to keep a type-interaction chart handy. Some of them are pretty obvious (Fire beats Grass), while others are... less so. (Dark is weak to Fighting and Bug.)

• Physical attacks (yellow starburst on red) use Attack, Special attacks use Special Attack (white ripple on blue). When a 'mon uses an attack that matches its type, its power is boosted by 50%. This is called the Same-Type Attack Bonus (STAB). Use this information when deciding what moves to use.

• For a rough estimate of different moves' relative power, multiply the user's relevant attacking stat by the move's power.

• Using the EXP Share (in X/Y and later) quadruples your XP gain and spreads it out among your entire party. As you might imagine, this tends to make the game very easy.

• Pokemon Amie will raise a pokemon’s affection, which in turn gives it an experience bonus, lets it spontaneously shrug off status effects, hold on at 1hp, and so on. There's not really any special trick to power leveling it. Striped and Rainbow Pokebeans are worth a lot of affection.

Game-Specific

• Fairy type is a new one. They’re strong against Fighting, Dark, and Dragon but match poorly with Poison, Steel, and Fire.

• TMs are now unlimited use

• HMs are out as of Sun/Moon. There are now “ride pokemon” you call to do HM-ey things. The moves still exist but aren’t unremovable anymore.

• Pokepelago (which you can access after the third trial) is now the place to grind out EVs, berries, and evolution stones. Save up your lil pokebeans so you can get everything except the hot springs up to level 3 sooner rather than later.

• Online stuff is now on the go with Festival Plaza. Local trading and battling, and online trading and battling are still around, and there’s some kinda poorly thought-out festival booth gimmick there too that you can probably ignore unless you’re into dyeing your clothes or whatever.

• Ultra Pokemon/Legendaries are mostly tied into the story (usually toward the end or postgame) and are pretty clearly telegraphed.