1) wheelies are easy, just do lots of loopouts and you'll eventually find that balancing point.

2) smaller bikes (bmx) are easier to initiate a wheelie than larger bikes (mtb). But it's easier to hold a wheelie on a larger bike because larger bikes give a wider range of balance.

3) doing manual is harder then wheelie. First master the wheelie, then progress to manual.

4) nothing beats practice. Go out and to it. That's how kids learn, by doing it. Adults ask too much questions on forums, not enough practice.