Bike slept in garage for too long.

I have a really old(6-8y) bike that i didn't use for like 3 years. The front wheel hub was broken, i recently changed it but haven't trued the spokes yet and idk how to do that as well. The main problem is i guess the gears not changing. Gears are pretty rusty now but i will try if a good cleaning might do the work. Idk how the gear mechanism works but what could be faulty about gears after not maintaining and using the bike for years? Do you think a proper cleaning can fix this chain and gears? I will lube everywhere needed but idk what else it might need.