As Salam Alykum

I was wondering I looked for the word RAA GII MUN . I went through the Lane dictionary. Under the words RAA Geem Ya but I could not find that word. I also tried Hans Wehr Dictionary of Modern Written Arabic. I seem to understand the root system a little but not a lot. As far as I understand the root system it works something like this if I was looking for a word lets say "Alif Seen Miim" (name) I would look under Alif Seen Miim. But the problem is some of the words are not there. I looked up more information on Arabic root words and this is what I found "Maktaba" (library, literature) would be under "k-t-b". How would I learn which word goes under which root? I do not know much arabic I consider myself almost a beginner. Can someone please teach me away of understanding this root system??? pr