Here's an article on how to invest when you don't have much money. The key is to get started and be consistent, even if the amount is small each time. It will grow over time if you are consistent and then it becomes exciting.
The main concepts here are to find commission free ETFs that by their nature are diversified because their underlying investments are across many different companies across different sectors.
Some examples of what a sector means in investing are financial, manufacturing, consumer goods, defense, energy, technology, etc.