There is no set rule as to what or what not to make notes of.
Some hard core spotters want to record everything that moves on the rails, including all the numbers of the wagons and coaching stock and you will see them speaking into small tape recorders quickly reading off the numbers of the stock as it passes, to later record into their books.
At the other end of the scale you get some who just photo and don't record anything in writing.
After a while you will get a feel of what you enjoy the most.
For me I have what they call a mole skin note book, they are more expensive than an ordinary one, but I like to keep a record of what I see and refer back to it over the years, so I like it to be a bit more substantial than most note books.
I record the location and date, and underneath the loco or unit number the time ,the headcode, where it came from and is going to, and the train operating company. As a result I spot both passenger and freight trains, but others only want to spot loco hauled trains, again you will find your preferences as time goes on.
The Platform 5 books mentioned are very good, but if you want to keep price down then this is also very good
http://www.anticsonline.co.uk/1692_1_107109737.html
There is a website called RealTimeTrains which is free to use and can be easily found on your phone which will let you know what is about to arrive and gives the headcodes for passenger trains but not freight. A good website for freight info and headcodes is Freightmaster but does cost £80 a year, and there is often a debate on here as to whether it is worth it, personally I think it is, and it compliments RTT very well, but again it is personal preference and often down to finances with these things.