if I multi-class into something, can I choose the subclass that I want, or is sub classes more for your original class choice?

Each class that you multiclass into will have a subclass (though you may need to be a certain level (1-3) to unlock it).

Thanks!!!

Remember also that Multiclassing is an optional rule, and your DM might not allow it.  Ask first if this was not already addressed in the Session Zero.

You will gain subclass abilities in both you original class and your multiclass if you level up enough.

A level 5 character that has taken 3 levels in fighter and 2 in wizard will have to have chosen both a Martial Archetype and an Arcane tradition.