One of the most common things a GM has to do when running a game is make rulings. The ability to make rulings is - in my opinion - one of the key things that makes a Table Top RPG infinitely more flexible than any computer game can be. See, every option before you in a computer game has to be specifically programmed for by the designers. If a player wants to do something that the designers didn't account for, well then they're just shit out of luck. But in a table top game, the GM can figure out a way to make it work.
So what should you consider when making a ruling? Let's go over the basic steps.
What Does The Player Want to Do?
The first thing you need to consider is what is it the player is trying to do? When asking yourself - or the player - this question don't just focus on the means by which they hope to bring about the end, but the end as well. There is a big difference between a PC trying to befriend the druid's wolf companions so he's on their good side, and a PC trying to do that in order to convince the local villagers that the wolves aren't something to be afraid of. And if you don't know what the player is going for, you may make the wrong ruling, or one that needlessly complicates things.
Does The System Do Anything Close To That?
The second thing is to check if your system does anything that is close to what the PC is trying to do. If nothing else you should have the system's 'core resolution mechanic' - the basis around how skill checks, combat checks, and all that are determined. But sometimes there is a more specific system or rules around things the player wants to do. If nothing else, a GM handbook may have some quick guidance if not full mechanics of something adjacent to the task.
This is a good idea to check just because why do work that is already done? And you may get ideas from for how you want to handle it for your game.
Can You Steal A Method From Another System?
If you have access to multiple systems, do any of them do the thing the player is looking to do? If so, can those rules be stolen if only for ideas? Taking from other systems is in general just a good idea as a GM. The 5e Legendary Actions mechanic can work in any system to make a boss feel more boss like. Aspects from FATE are easily stolen to give and track penalties and bonuses in a situation or to even tag or invoke on players and scenes as needed.
Is This Ruling Liable To Become A Rule?
The difference between a ruling and a rule, is that a ruling is meant to resolve something in the moment so that normal play can resume and the game can continue. A rule on the other hand is part of the game. It will be referenced again. It will come up again. A player wanting to make a magic sword as a one time thing is a ruling. A player wanting to make magic swords as a thing their character does on the semi-regular? That's a rule (or rule set.)
The more likely something is to be a rule, the more complex you can be with it as needed - provided it works, is fast enough for your needs, and is fun. The less likely it is to come up again? The more I suggest you stray to a simple method that gets the job done and move on.
Make Your Ruling
At this point, in most circumstances, you should be able to make your ruling. This may seem like a lot, but in the moment at the table it goes by very quickly. And when given time to make a decision you can more slowly indulge in looking for rules.
Don't get hung up on looking up rules at the table. When in doubt, make a ruling and move on. You can always look it up later and adjust as needed from there. But do consider what you know of the system, and how it works.