Always good to see someone new get interested in birding. I am sure the more you learn and the more you get out there and see new birds the more the bug will bite
The very first thing is to invest in a proper field guide such as the Roberts 7 or Sasol birds of Southern Africa. They are even available on iTunes and Android market.
The best method of learning, and I will stick with this, is to physically get out there and go and look at birds with someone who is fairly experienced. This is the best method to pick up the various methods, tricks and generall tips on how to make an ID on a bird.
There are various factors to take into account when it comes to big brown Eagle's. If you have a field guide you need to factor in various things such as size, which you mentioned, colouration, habitat, range/distribution and whether the bird is a migrant or local
Wahlberg Eagle's are definitely smaller than Tawny Eagle's. Wahlberg Eagle's are summer migrants to SA so if you perhaps saw a big brown eagle in July, then most probably a Tawny. This is all just an example of what to look for etc. There are other things such as colour variation too and general look and feel of the bird.
There is a general term in the birding world called "giss" or "gizz" or whatever but this is the term used to described when you have gathered enough experience or knowledge of birds that when you see for example a big brown eagle the overall look and stance etc of the bird immediately screams a certain ID inside your head.
Birding is a fantastic, fulfilling, relaxing hobby that takes time to get good at. Like any other hobby you need to practice, you need to get out there, get a book, take some photo's even if it is in your neighbourhood because you will be surprised at how many species live on your doorstep.
Post your questions here as there are some great birders with a lot of experience and knowledge on this forum that can help you get better and enjoy the hobby
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