You are really asking two questions here:
1: How hard is for an european IT professional to get a job in the USA?
Same as anywhere else. Depends on regional factors, industry, your skills and level of experience and - most importantly - what connections you have.
2: Visas and work permits
I started down that road a few years ago. Gave up as the red tape was just killing me. Also, the idea of basically being indentured to my employer for 5 years seemed less and less appealing. I hear they have loosened that up a bit though.
The H1B work permit and green-card quotas for employment-based immigration gets wildly over-subscribed each year so you need both luck and timing. The H1B application window opens April 1 so you need to have your application ready and in before then to even have a chance and it'll still be a lottery since every year the number of applications will exceed the quota on the first day already.
For a green-card, the waiting period is measured in years for EB3 (professional/employment based immigration) so unless you secure an H1B work permit (under the dual intent model) before applying for a green-card, you'll need to find an employer who can sponsor you with the promise to hire you in 6+ years from now (not super-easy). It can't be a hollow promise either as the employment will be follow up on by the government and if it turns out that the job is not bona-fide, you can get your green-card revoked and be deported.
Depending on the situation, your employer might also have to obtain a certification from the department of labor where they have to demonstrate that they are unable to find an American to hire for your intended role. Securing this is very dependent on where in the US you are and what industry you are in. Finding a certain skill-set might be very easy in one place but not in another. Regardless, your employer will be making a sizable investment in you, both in time and effort and you will, likewise, be very dependent on that employer for a number of years so unless you have connections or know some potential employers, it might not be that easy to get the required support.
As for everything else in the US, this is not something that you will want to do yourself There are lawyers specializing in immigration and either you or, preferable, your employer will need to retain one to navigate the maze of red tape, forms and bureaucracy that is US immigration.