I entered USA on VWP. I have a H1b but it will only starts in a few weeks. Can I go to Canada and change status (I am EU citizen)?
Yes, you can.
When re-entering the US just make sure that they are aware that you have a visa and are entering using that rather than the VWP. (Now that there are no longer I94 forms when entering by air, it's worth mentioning this every time you pass through immigration - especially given that you likely also have an active ESTA)
Presuming you are planning to travel from Canada to the US via air, keep in mind that you will pass through immigration in Canada before boarding your flight. Although a first entry on a H1B should be quick and relatively painless, it would be worth making sure that you arrive at the airport with extra time just in case there are any problems.
I have done this myself in the past, although in my case it was moving from one visa to another, not from VWP - but the process is the same.
Note that an Adjustment of Status (AOS) as mentioned in one of the comments is NOT an option in your case for a number of reason - the most important two being that an AOS can take months to occur, and even when it does occur you will not have an I94 which will mean that your employer will have difficulty confirming your legal status.