Learn Spanish (Latin America): Salgo al parque temprano.

Salgo comes from the verb salir (to go out or to leave). While most forms of this verb follow the normal rules (like tú sales or él sale), the yo (I) form is irregular. Instead of being "salo", it adds a "g" to become salgo.

Where is the word for "I" in this sentence?

In Spanish, you can usually drop subject pronouns like yo (I) because the verb form already tells us who is doing the action. Since salgo can only ever mean "I go out," adding yo before it is completely optional.

Why is it al parque instead of a el parque?

When the preposition a (to) comes right before the masculine article el (the), they must combine into the single word al. So, a + el parque merges to become al parque (to the park).

If salir means "to leave," could this sentence mean "I leave the park early"?

Not in this case, because of the preposition after it! We use salir a to mean "to go out to" a destination. If you wanted to say "I leave the park" (as in, departing from it), you would use the preposition de (from) instead, making it salgo del parque.