Questions about Attack on Titan (May contain spoilers)

1. Who created the titans or what are their origins?
2. Are all titans humans or are there natural titans?
3. Who is the strongest titans?
4. Is the population inside the wall what is left of all of humanity?

1.

Spoiler:
Titans are descendants of Ymir. Ymir Fritz was the founding Titan. As far as the story tells right now, she's the first ever.

2.

Spoiler:
Only Eldians can become titans. Every titan seen so far is an eldian.

3.

Spoiler:
The founding titan. Has the ability of scream which can command any and all other titans and could even wake every titan inside the walls.

4.

Spoiler:
Nope.

Thanks so much!