What to do

I welcome anyone to contribute, and you can work on any aspect you prefer. If you need ideas on what to do:

Check out the issues section. I regularly open issues myself to keep track of them and also for others to tackle them.
You can also join our Discord server, a growing community of 250+ members, where you can ask for ideas on what to contribute.
The readme page has a roadmap section, which is another way to see how you could help.
Another great approach is to simply run the engine, see what doesn't work or could be improved.

General

Try to adhere to the KISS principle whenever possible.
Avoid defensive programming. Instead, use asserts everywhere (offensive). If an assert pops up, it should be fixed ASAP.
If auto is used, the type must be deducible at a glance. Long iterator types can also be replaced with auto.
Choose the right tool for the job. Refrain from dogmas like "Only use orthodox C++" or "Only use modern C++".
You can use tabs or spaces. The project will replace all tabs with 4 spaces to maintain a consistent look across IDEs.
Use comments, but if you can name your code well enough that it doesn't need to be commented, that's ideal.
If a class doesn't need to have multiple instances, make it static. Don't invite unnecessary complexity.

Variables should be named using the snake_case naming convention.

bool name_like_this;

It's closer to natural language and easier for our brains to read.

When you are initialising a lot of variables, try to align them, in a symmetrical manner, like so:

m_matrix       = Matrix::Identity;
m_matrix_local = Matrix::Identity;
m_parent       = nullptr;

A Visual Studio extension that automatically aligns like this can be found here.

If statements should always have brackets, the brackets should be on their own lines, like so:

if (condition)
{
    // code
}

The only exception is the early exit if statements, which can have no brackets, like so:

if (condition)
    return

Try to use const for function parameters, if needed. But don't worry too much about const correctness.

Clone/fork, work, submit a pull request.