There's a guide with lots of tips in Official Guides => General
But most importantly these:
1) EV assigning is manual in PBO, you have 260 EV's that you can train on accident (meaning to get 252/252/4 EV's assigned, you can train an extra 260 EV's but can't assign those, to make sure small mistakes in EV training arent forcing an EV reset item.
(more info in Official Guides => General => Pokemon Stats)
2) VS. Seeker after gym badge 2 (after bike bridge) is a heavily underestimated content for starting players. It makes buying pokeballs less needed & gives pretty good Experience every 2 hours.
(more info in Official Guides => Sinnoh => VS. Seeker)
3) PBO is harder than your average pokemon game. Don't give up & try to find strategies to beat gym leaders, dont always just overlevel as that will quickly feel very grindy.
Example: multiple Intimidate shinx's to lower the AI's pokemon stats to a point your actual story pokemon can beat the pokemon (or setup themselves to beat all of the opponents pokemon).
Example 2: Butterfree with sleep powder & quiver dance
Example 3: a low level pokemon with access to Rapid Spin clears hazards & might still be slower than the opponent on +6, removing the hazard constantly & forcing ai to set them back up untill the hazard setter dies
etc etc
4) Have fun & ask around in chat, vast majority of the players will help you out when they aren't too busy grinding shiny/ur pokemons
