you could start by analysing the requirements and identify what the application should do.
No requirements? Speak with Technical Responsible and ask the most important features.
Write down tests for it.
The software only runs on the target hardware or can you run it on a desktop computer?
Depending on the answer, determine how is the best way to interact with it, to send inputs and get outputs back.
On other words, what are software's public interfaces, visible from outside? Your functional system tests should cover them all.
Then you can choose a testing tool that interacts with the software.
Determine if test results can be automatically or manually checked.