What resources can I use for learning character decomposition?
you may try the 拆字字典:

http://www.kaifangcidian.com/han/chaizi

eg:


【彎】 wan1    〖絲 言 弓 彎〗〖糹 言 糹 弓 彎〗


http://www.kaifangcidian.com/han/chaizi/彎

have fun :)