the online test is similar to other analytical/logical reasoning tests. So you can expect to have a set of 20-30 questions where you'll have to complete patterns. So they provide you with a sequence and then you need to select your answer (out of 4 options) on how you thing the pattern continues (could be colors, squares, or anything of that kind).
Best way to prepare for it might actually be here on preplounge and the resources provided! As it doesnt differ much from any other logical reasoning test you can however also find plenty of examples elsewhere or with other companies.
My advice: Don't worry too much about it. If you've done the GMAT for example you will find the Innogy test extremely easy. If you haven't done the GMAT, just revise some examples to get an idea of how these things work.
Let me know if you have any other questions.