When you say combine, you mean a compound like 人口 ? You can't (always) think of it as "person mouth" as that makes absolutely no sense. In (I'd say) most cases, you really need to learn the vocabulary. There are some kanji that go together quite intuitively... but there are tons that don't.
My best and only advice as a beginner is flash cards for vocabulary... and lots of them. Also read the sentences in your texts (online only will not suffice). Once you get some vocab down, use them in sentences to get a feel.
Is Japanese a hard language? Yes, at the first, it's like pulling teeth. the kana, kanji, synonyms, levels of politeness, and all around weirdness is a lot to absorb. About 3 years into it, you'll get better, and eventually it will click. You'll eventually get to where classes really don't teach you anymore than putting your nose in a book can assuming you have a lot of practice to go with it.