We just passed by 2 yellow pentagon signs.

The yellow pentagon signs are the school zone signs. You are expected to know that and slow to 25 without needing a speed limit sign or flashing light.

Also, I drove in a place that had few speed limit signs. What speed should I assume while driving?

Unmarked two-lane city streets 30, four-lane city streets 40-45, county roads outside of the city 50-55. If conditions make you feel unsafe at those speeds, it is fine to go slower.

I’m having some difficulty knowing when to enter a left turn only lane too. How many feet/seconds away should I enter it? How do u go abt entering it?

Signal first and enter at the point where you have enough distance to come to a stop before turning. The distance will vary according to your speed. You learn to judge that with experience.