Learn Italian: L'uomo è qui.

Even though uomo is a masculine noun and normally uses the article il, Italian drops the vowel and adds an apostrophe when the following noun starts with a vowel. So, il uomo becomes l'uomo to make it flow better when speaking.

Is uomo a masculine or feminine noun?

It is masculine! As you are learning in this lesson, Italian nouns ending in -o are generally masculine, while those ending in -a are feminine.

Why is there an accent mark on the word è?

The accent mark is very important. è (with the accent) is the verb meaning "is" (from the verb essere). Without the accent, e means "and". The accent changes both the pronunciation and the meaning!

If I wanted to change this to just say "He is here", how would I do that?

You could say Lui è qui, using the pronoun lui (he). However, Italian often drops the subject pronoun. Simply saying È qui is perfectly normal and usually the most common way to say "He is here".