Do i have to file all forms for immigration together or send form by form starting with I-130 ??

Outside the US you start with the I-130 and continue later with the NVC.

Renew work permit

The first has no charge, renewals have the normal filing fee.

My girlfriend is Canadian citizen. I have US green card. If I marry her, will it ever be possible to become Canadian Resident

There is no US immigration prohibition to obtaining Canadisn citizenship.

What is the best way to get my husband a work permit or permanent residency without him leaving the country?

Your husband's manner of entry to the US and some other information is needed to determine if resident status in the US is possible without leaving.

How to ask to stop working when a case is not yet opened

Just let the attorney know. You can discuss what charges if any with the torment.

Can I travel to home country when change of status in progress

The change of status will be considered abandoned and denied.

Can I travel through Europe to India when visa in progress

You need a valid visa to return to the US. Your H-1B may only be considered valid if you have been maintaining H-1B status and the visa has not...

Can I join company C ?

You should speak with the attorney from Company C and have them file for a change from B to C

How can I get green card ? I have only visiter visa !

Employment or family petition are the principal means to resident status. Talk with an immigration attorney to learn if you have other options.

You will require a new LC and visa petition but not the same pd to port the priority date to a new employer.