speak is more formal than talk.
speak can be one-sided,
i.e. speaking at a conference = making a speech at a conference.
talking at a conference = possibly having a conversation.
everybody was speaking all at once = nobody was listening to each other.
everyone was talking = groups were having conversations.
What you need to know first is the endings of all the declensions. These should be committed to memory. First declension nouns are almost always feminine. Second declension nouns with "-us" as the nominative ending are masculine, while those with the nominative ending "-um" are neuter. Fourth declension with the nominative ending in "-us" I believe are generally masculine (but sometimes feminine), while those with the nominative ending "-u" are neuter. The vast majority of fifth declension nouns are feminine. The hardest one to determine is the gender of third declension nouns. For the third declension there is no rule of thumb to determine the gender of a particular noun, you just have to know it.