From my experience a pad a pen is a good start, but you'll be graduating quickly to something much larger.
After a good 10 years away from this hobby I wandered back in in 2016 and found myself at an auction in a small town about 2 hours away. The large collection of a long-time buff was being auctioned off after his passing. I looked around the room and saw a few people like me, but I noticed mostly what I would call "serious enthusiasts".
A number of these folks were carrying 3" or 4" binders, which turned out to be books holding their wish lists and related details. One fellow actually had two volumes for his.
Aside from being intimidated, because my list was a mere 8 items long at that point, I decided to find a more modern way to build my version of the binder/set of binders.
On that day, October 29, 2016 I started an electronic version with a single entry, using an application called Evernote. As of this morning there are now 7125 more entries, and Evernote has come to consider me a 'power user'. Yet I can access any item in my important items list from my phone, anywhere, and at any time, as well as my laptop. There is now no need for me to carry any paper whatsoever.
In the middle of this screenshot you'll see a list of 10 of those 7126 entries, a short subset of the entire database. To the right are the details for one of the 10, the highlighted one, including text and pictures.
Pay careful attention to the list in the black column to the left as it contains a special list of keywords, many more than you can see. The secret to Evernote is the ability to add keywords to each entry (like most software), but it also carries the relationship between those keywords. This is very, very powerful for finding things later.
I started this effort to make sure that I'd never forget anything anymore, and I now don't -- as long as I don't forget to enter it. When I go to a train show, or an auction, or while surfing the web for deals, all I need is my phone.
Good luck with your new effort. Being organized is helpful. Whether you use paper or software O Gauge/Scale will bring you many hours, days, weeks, months and years of enjoyment.