Is there a transition period before joining H-1B employer the first time?

There is no such thing as transitional period. The H-1b sponsoring employer is obligated to pay H1-B beneficiary upon the start date indicated on...

I tried to cross the Canadian border and denied .I just married a girl from Toronto can I cross the Canadian border now?

probably not.

Iam a US Citizen who wants to marry an immigrt from Guatemala. can I apply for permanent residency for him after we get married

It depends on facts that you have not mentioned in your question such as, his last entry into the U.S. If he entered the U.S. in legal status...

Can I meried again I am in process of removal ,my wife find another man don't know what to do ,also I have somebody .

You have some real issues.

I-130 transferred to Nebraska, still on process

You just need to wait for notices from USCIS.

Do I have to file for adjustment of status, right after my contract for my internship ends or when my visa expires?

You can file immediately if you are eligible to adjust your status to an LPR (Green Card holder). There are many missing facts. For instance,...

How much time does it take to get green card after PD becomes current ,

Normally, 4-6 months.

Change of Status reg.

You should seek assistance from our employer's attorney.

Problem. Call an experienced attorney ASAP.

Regarding Green Card & L1A extension filling

Ask your actual lawyer.