Hi, great that you want to contribute!
The easiest and probably most rewarding way to get started is by porting a new board to RIOT. That way you get to try out most low-level features when you verify that it’s actually working and may see areas where more work is needed 
See this post for a list of suggestions, but mind you that the posts are not kept up do date. Some boards there are now supported, althought not all drivers might be there yet.
In case the microcontroller / family is already supported, porting a board should be pretty straightforward based on existing boards.
If you add a new microcontroller family from a vendor that already has support, peripheral drivers need to be adopted and clock setup is usually family specific. So it’s more work but also a more interesting task.
When adding a whole new vendor, you will also have to provide peripheral driver implementations. We recently added support for RP2040 and GD32VF103, but they still lack most drivers - this would also be a good task to start.
If you want to start with something simple, you can also do a sensor driver for some I2C or SPI based sensor.