An or A?
This is a small, but important point of grammar, and one that will elevate your grammatical style. You should remember that a word that begins with a vowel should be preceded by an (e.g. an apple). A word that starts with a consonant should always be preceded by a (e.g. a lightsaber). Note that with words starting with h, it is now acceptable to use an or a e.g. a historian or an historian.
Susie and I; Joe and me
Using these constructions correctly will help your writing, and often the speaking of your characters, to sound a great deal cleaner and more correct. Me and I are used different situations.
I is the subject form of the pronoun so: Keldorn and I decided how to grade the Krath Grammar Studies exam.
Me is the object form so: Ronin gave Keldorn and me medals.
A simple tip to decide which to use in most situations is to remove the other person's name and just focus on the me or I. For instance, in the above sentence, remove Keldorn and. Now look at the sentence. Which is correct? Ronin gave I medals or Ronin gave me medals? You would never dream of saying the first. The fact that there is another person involved does not change the rule. Simply add in the other person without changing the personal pronoun and the sentence will be correct. Note that the personal pronoun should always come last, an easy way to remember this is that it is a matter of politeness: always announce others before yourself.
One very common example that you need to remember correctly is this: Ronin shared the ammunition between Keldorn and me. Never write or say Between you and I.