First of all, based on that you're a new player you might not know it but as Castille you never need to fight Aragon. Because there is an event that almost always fires(if it doesn't fire then you're VERY unlucky) that basically puts Aragon in a PU under you. So just ally them if they accept or improve relations so you have good relations so that you never fight untill you get the PU because conquering them would be pointless and would make you waste resources.
As for France, if you want to play a colonise oriented game, just ally them and have fun winning all land wars with no effort in Europe unless they start unliking you for some reason. If they decided to rival you(which isn't very likely unless you expand a lot in Europe) you might just ally Austria unless they rivalled you too... If you want to conquer France though, you should be quick about it. You should have Burgundy(if still alive) and Austria as allies and beat them before they get their 3rd idea. Just release everything you can out of them then conquer the little pieces one by one, or diplo vassalise some of them then diplo annex as you choose.
As for Portugal, it's probably the most profitable way to just conquer them before colonisation starts since they will be a competer for colonising... Possibly after you've established your PU with Aragon so that they both don't attack you on two fronts. But I actually prefer letting them live since I like having them around and having to compete them while having great relations. Also if you want to conquer North Africa, they usually join those wars.
Though you'll probably conquer Portugal too if you had already dealt with France.
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