Hokkaido on August

Hi friends
I am Jane. I live in Niigata. I have planned to visit hokkaido this obon festival.I still have many questions about it. WOuld you mind telling me?

1. I would like to go to Sopporo by ferry from Niigata port. What kind of ticket should I buy?..Cheap wihtout bed or It should have bed.

2. I wish to back from Hokkado to tokyo by night train. I understand that there are a train line which understand water between Hokkaido and Honshu. Does it correct? I hope I have that experience under water :)

3. Can I use Seishun Kippu at Hokkaido? and Can I use it with night train to tokyo?

4. Should I rent a car at Hokkaido? I understand that the best way to travel there is car...I have planned to visit Lavender farm too

Sorry for many questions
Thank you very much for your time

I would like to go to Sopporo by ferry from Niigata port. What kind of ticket should I buy?..Cheap wihtout bed or It should have bed.

This is entirely up to you. I personally would prefer to pay more for a private room with bed:
http://www.japan-guide.com/e/e2355.html

I wish to back from Hokkado to tokyo by night train. I understand that there are a train line which understand water between Hokkaido and Honshu. Does it correct?

Yes, that is right. The trains are called Hokutosei and Cassiopeia. Here are more details:
http://www.japan-guide.com/e/e2356.html

Can I use Seishun Kippu at Hokkaido?

Yes, on all local and rapid trains on Hokkaido. But not on express and limited express trains, as explained here:
http://www.japan-guide.com/e/e2362.html

Can I use it with night train to tokyo?

No. The night trains are classified as express or limited express trains.

Should I rent a car at Hokkaido? I understand that the best way to travel there is car...I have planned to visit Lavender farm too

For Furano and other rural areas, I recommend a rental car. But inside Sapporo, public transportation is good.