Professor Super gave an outstanding theoretical discourse on collecting. I would just add what it looks like in practice, at least in my case.
When I was little, I didn’t have much of a choice. There was no internet, I didn’t even know what all there was in the world. But I knew what I wanted. My dad who travelled the world knew he wouldn’t go wrong if he brought me some tram or train because he knew I loved them so much.
Now that we have the internet, and can see everything the world has to offer, it’s often hard to decide what we actually want. There is a difference between what we like and what we unconditionally have to have. For example, I frequently see beautiful layouts and wonderful trains and trams of all kinds on YouTube, but I won’t start collecting them just because someone else has it and because someone collects it.
But when, for example, it was announced that Plarail had released the Shin-Keisei 80000 commuter train as a limited edition, something sparked in me, I immediately started looking for that train (although they are not very easy to get because these limited editions are mostly sold in the sales network of the respective railway companies), found them, bought two of them, and in two or three weeks they were on my layout. And wow! Even though I’m 54 now, I was happy like I was 12 when they arrived, I played with them all night. And the whole next day.
So just wait until you feel that spark in you, which will tell you what to do about collecting, and you certainly won’t go wrong. 🙂