Movepool and stats are the two most basic ones to know. Check them out on the db or bulbapedia in the generation you're playing in. Take Salamence for example. It has access to dragon dance and high offensive stats but low defensive stats. This means that it's supposed to sweep because most of the time, it won't survive a stab super effective hit. Another example would be Blissey. High hp and special defense means it is a special wall and it has a lot of supporting moves like wish, light screen and heal bell. Some Pokemon can perform multiple roles such as Garchomp so in that case, you need to select moves based on what you want it to do. For example, if you want Garchomp to be a sweeper, you need to give it choice scarf and four attacking moves. If you want it to be a nuke, give it outrage and dragon z crystal
So bottomline is, analyze the movepool and stats of the Pokemon you wanna use and take it from there
