Hi Raimundas and welcome.
Where to start?... Broad question! It depends if you have a home plan, or inner or external measures, levels or not, the degree of precision you need, etc... In Documentation, the User's guide is much helpful, have a look also in "Tip and tricks" where you will find some tutorials ("How to design rooms from inner dimensions" for example) and articles. There is also some special beginners film...
Good luck and see you soon in the gallery!