Petr,

Welcome! Sounds like you have a big project ahead of you. Please note that it is worth searching the forum for your questions, as this very same question has come up before in discussion here in various formats. For instance:

http://socialsounddesign.com/questions/618/tv-broadcast-levels
http://socialsounddesign.com/questions/3254/dia-always-the-same-level
http://socialsounddesign.com/questions/6114/tips-for-mixing-dialogue

There are so many standards thrown around on the internet for what level you should mix to, but ultimately you need to experiment and learn for yourself. Experienced mixers go off of what they hear in front of them, and base many of their level-setting decisions on previous mixing experiences.

Ultimately, calibration of and time spent with your monitoring system is your most important ally when you mix. If you set up your speakers so that the levels you mix to are directly corollated to the levels you hear then you are free to make creative choices without worrying if your dialog level is hitting __dB. Even more, if you calibrate your system based on the destination of your film then you give yourself a huge foot forward. If your movie will be shown on youtube, you will calibrate your system differently than if the movie will be shown in a theater, which is different than broadcast TV, DVD, etc... Watch this movie for more detailed instructions on how to set up your speakers:

http://vimeo.com/22735507

I also must recommend finding an experienced sound designer to take a look at your film. This is why we exist! Even if it's just a 1- or 2-hour session, getting a demonstration on the mixing process might make a huge difference for you when you sit down to mix. It could be well worth the time and money.

Best of luck!
~Matt