How to get small numbers (For example q0)

What you're looking for is subscript and you write it like this: $q_0$.

Note that this only works in math mode. The $ tell LaTeX to switch from text to math mode or back (there are different possibilities of switching to math mode, see here).

When you want to have multiple digits (or other symbols) in subscript, you will have to enclose them in curly braces (q_{12 + x}), since the _ only "sees" the next symbol. The curly braces make LaTeX se everything inside as a group that can not be separated.

If you want to use subscript in normal text, have a look at this question.

Superscript works the same way but with ^ instead of _. Write x^2 for x².