Learn Croatian: On je ovdje.

No, you cannot start a sentence with je. Because je is a clitic (a short, unstressed word), it must follow the second-position rule. If you want to drop the pronoun, you have to swap the word order so je comes second: Ovdje je.

If we can just say Ovdje je, why do we include On in this sentence?

Croatian usually drops subject pronouns because the verb already tells you who is doing the action. We include on (he) here either for emphasis (meaning he is here, not someone else) or to make it clear exactly who we are talking about if the context isn't obvious yet.

How are On and je pronounced together?

Because je is a clitic, it has no spoken stress of its own. It leans on the word before it. When speaking, you pronounce On je smoothly together almost as if it were a single word, with the voice stress falling on On.

I've also seen the word tu used for 'here'. Can I say On je tu?

Yes! Both ovdje and tu mean 'here' and you will hear both in everyday conversation. Strictly speaking, ovdje means right exactly here where the speaker is, while tu is slightly further away (near the person you are talking to), but beginners can use them interchangeably in most situations.