Can a muslim man marry himself
First of all you should know that a Muslim man can marry himself without any waly even if his parents do not approve!
But he should choose the right woman as our Messenger (peace be upon him) told us how or which one to chose (see for example this hadith narrated by Jabir (May Allah be pleased with him) or this one narrated by Abu Hurraira (May Allah be pleased with him))-> a religious woman/girl.
Now assuming you did the right choice you are free to disobey your parents, but if you could chose any other woman/girl which your parents would accept this would be the best way! But in any case you should try to find out for which reasons your parents reject your marriage and try to explain your point of view! And as @servant wrote in his comment you should try to convince your parents either yourself or by mediation of a relative or friend who could convince them!
If your choice is bad the best would be to listen to your parents and look for an other girl/woman to marry which would be a good choice according the hadith mentioned above!
If I commit Zinna, will I be sinful? and what about my parents? will they be too?
Committing Zina is haram (as anything leading to it could be considered as haram) and it doesn't matter whether your parents haven't let you marry whom you want or not if you committed this sin it's on your responsibility (see in the Quran for example in Surat al Baqara (2:286):
It will have [the consequence of] what [good] it has gained, and it
will bear [the consequence of] what [evil] it has earned.
or in Surat al Mudathir (74:38).
So Zina is not and should never be an option for a believer!
Finally here's a fatwa to make clear that you could marry yourself! And another one explaining that if you did a good choice and your parents refuse this would be a mistake on their side and some advice to prevent the big sin of zina!
And Allah knows best!