In addition to all of the rest of the things you have to worry about for your pokemon’s EV points, you cannot forget the most important decision you have to make- where to assign them. You have to actually make the choice of where you want the EV points on your pokemon to be, or understanding how they work and how to get them isn’t going to mean anything.
And that’s where a bit of strategy comes in. By this point, I’ve gone over the types a pokemon can have and its attack types. And I’m sure you have at least a basic understanding of stats. So the issue really is twofold- what do you want your pokemon to do, and how do you want to make it ready to do it?
Personally, I like to take what I consider a balanced view- I most often reserve half of the EV points I put into most pokemon for shoring up a weakness (Magcargo’s terrible Special defense, for instance), and the other half I usually reserve for making the pokemon better at what I want it to do (In that same case, my most-used Magcargo has the rest of its EV points invested in its Special Attack stat).
There are other ways to do it, and they really depend on the actual stats of the pokemon in question. Bastiodon, for instance, has a terrible speed stat (and Magcargo isn’t so great at it either). Now, I could invest my EV points in boosting that, but the fact of the matter is that the 63 stat points that would actually come of a full set of 252 EV points is not a big enough amount to make it worth the effort- it’s still going to be slower than most of the things out there that I would care about whether or not it can outspeed.
Similarly, there’s a point of excess in investing your EV points to make a pokemon better at something it’s already good at. Bastiodon has huge Defense and Special defense stats, and only mediocre Health Points. I -could- invest in its Defense if I’m worried about physical attacks, but that’s simply not going to be very effective because its Defense stat is so huge. I’d be better off investing those points in the creature’s HP- and thereby boosting its protection against all attacks.
This isn’t always the best way to do it- there are some pokemon so good at what they do that your best act is to invest in shoring up their weaknesses (vis a vis Rampardos, whose Attack stat is so ridiculously huge there’s no reason to make it better) and pokemon who are so overall stable that you only want to invest in where you’re specializing them (Such as Venusaur, whose stats are all around pretty good but have only a modest leaning towards special offense and no actual weaknesses).
Keep these things in mind about EV points and you’ll be in a good place- though you’ll want more specific information, which can be found in later ‘courses’.