I've been learning Japanese with Memrise's program (for about 6+ months at 2 hours a week on average) and have managed to learn all the hiragana characters. They don't teach katakana (I guess they assume that all words using katakana are English cognates that I'll automatically understand what they mean?) and move straight to kanji.
This is where I struggle, especially with characters that combine multiple kanji into one. My understanding is that less common kanji are accompanied by furigana (hiragana that are placed in small print over kanji to help others pronounce the character), but I can't quite understand whether or not a word is, by standards, "uncommon" and if I should devote as much time to words like these where I'm not expected to remember how to pronounce them.
How can I better assume which words will be paired with furigana without personally experiencing these words on the streets?