My corporate culture

I want to present some aspects of my company's culture.
First, we have a very supportive culture. New employees get a mentor, which [who] coaches them in their job and helps them doing [with] their first steps in the new team. We have well defined targets in our team, so we all know, where it is going and what the most important aims are.
Also we have an informal culture. For example, there is no dress code in the office and we can talk directly to our leaders if we have any requests. Employees of nearly all hierarchies ['at all levels of the hierarchy'...note: there is only one 'hierarchy'] share an open plan office. Sometimes it is getting [gets] a little bit loud in our office, but if we need to work very concentrated ['concentrate'], it is also possible to book a small single office for some [a few] hours.
For me, it is a good place to work, espaecially because of the good teamwork and communication.

Note: You write well, so I also put in some stylistic comments.  For example, it's not really wrong to say 'work very concentrated', and you would be understood, but that's not how a native English speaker would put it.