I can't build!!!

I am very new to League of Legends and have read somewhere that when you build, you should build differently for different situations. I know there are websites like Mobafire that give you different builds, but I want to learn to build without using any websites. I think there are some rules to help identify what to build. I have seen players building differently while playing the same champion and I always wondered why? and how? I will be really happy if someone could help. Thanks, ReallyDeally
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