I'm new to Bonsai trees, and recently bought my first one. Although after about 15 days the leaves have begun to fall off, and there are strange white web-like spots on various branches, and I'm not sure what to do about it. (Also, in case anyone's wondering, I am buying a new pot to replant it in, as well as proper soil and fertilizer within the next week, the current pot is the one it came in, and I am aware thsat it isn't ideal)

Hello, welcome to the forum.
Could be wooly aphids or mealy bugs. Both are not so difficult to get rid of.
Please search this forum and elsewhere for insecticides or ways to kill them.

Do NOT repot your Ficus Retusa untill it is fully healthy. Repotting now may kill it.

Get rid of the aphids. Part of the process is to move it outside for summer in a brighty lit spot.