This is a two part question. The answers might be obvious but I'm asking because it's not to me. :(
If my first class is able to use a shield but the class I multi into says that it's not proficient, can I still use shield because I got the proficiency from the first class?
If I multi in to for example agent of the grave that says, "Agents of the grave gain no proficiency with any weapons or armor." Does that mean I'm just not adding proficiencies or does that mean that I no longer have my other class(es) weapon and armor profs?
You do not lose proficiency in weapons or armor when you multiclass. You might not be able to use certain things with abilities from another class, but you do not lose the option.
For example, taking a level of fighter gives you heavy armor proficiency. Taking a level of wizard gives you no armor proficiency. Your character could still wear heavy armor without taking penalties to his attack rolls because his fighter level(s) still make him proficient. However, because wizards can't cast in armor without having to deal with ASF, it might not be a great idea.
When you multi class, you are proficient in all weapons, armor, and shields that any of your classes are proficient in. The only issue you can run into is with arcane casters. If you multiclass fighter and wizard, you are proficient with all simple and martial weapons, all shields, and all armors. But, if he is wearing armor or using a shield, he still has arcane spell failure chance.
Thinks guys for the fast replies :D
I'm a lvl 3 fighter and very bored with just wacking things so I want to multi into something else for some more diversity in attacks, since I'm not allowed to make a different character