A noun is a person, place or thing ... so which of these words are a person, place or thing?
First, the runners wait for the signal to start.
It can't be first because that's not a person or place or thing. The is just a modifier, so none of those are nouns. Runners are people so that's a noun. Wait is an action word, a verb. For is a linking word so that's not it. Signal is a thing so that's a noun. To start is another verb.
The first word of a sentence must start with a capital letter. There are millions of ways to start a sentence!
First think about what you first know about the subject
It's not generally done, no.
you say first, because firstly is not a word
Yes.
Only if it's the first word in a sentence.
No, unless it is the first word of a sentence or part of a proper noun.
One way is........
Sure, as long as the beginning word has a capitalized first letter.
No, "first trustee" is not capitalized unless it is used as part of a title or if it is the start of a sentence.
Always start the beginning of a sentence with a capital letter.
When a sentence is enclosed in parentheses, you typically do not need to capitalize the first word unless it is a proper noun or the start of a new sentence within the parentheses.