Every pokemon can unlock 2 abilitys. These abilitys are different for each pokemon so if you want to find the best abilitys for you pokemon you will have to search for a bit or ask other people.
When you just get a pokemon it will already have a first ability. The second ability has to be unlocked. To unlock the second ability the pokemon will need to meet a few requirements: Monster friendship level has to be 70 (meaning that this is only for level 70+ pokemon aswel), Fusion level 20 and the total attribute score has to be 600. Once you meet those requirements will also need some materials. The materials can be found in master dungeons. You will need 60 stardust, 30 star pieces and 15 ability capsules.