Todo mundo: everyone gets confused over these two common words. Actually, they're called indefinite pronouns and their job is to "represent either count nouns or noncount nouns". What that means is not important. These are just the words that convey, all, every, entire etc. The confusion stems from the fact that tudo sounds a lot like todo.
To keep them straight you just need to think of tudo as everything. Tudo in portuguese is used when talking about non-specific things.
So for example when you say tudo bem, you're saying everything's fine. Tudo bom = everything's good. There's no gender to it because you're saying that tudo is bem - the weather, your job, your mom, the president and the world in general. Tudo.
If it's everything in English, use tudo.
Of course you get into a very grammatical discussion about countable and non-countable nouns but let's not. Tudo has no plural - it itself speaks of - everything, a sort of plural!
Use todo/a to express all, every, entire.
The plurals are todos and todas. When you use the plurals you needs to include an os or as. Like this:
Todos os meninos estão com fome.
Todas as garotas foram à festa.