DisneySea questions

Will it take me a full day to Disneysea?
What kind of ticket should i buy if i would like to visit from day to night?
Which days are the less crowded day to visit disneysea, does weekdays has lesser or more visitors?

If you are wanting to visit every area, see all the shows, and ride most of the rides, then yes, you will need at least a day to do Disney Sea.

You should buy a 1 day passport if you are planning to only visit Disney Sea, and only for 1 day. They also sell other kinds of tickets if you have more to your plan than just Disney Sea. Don't forget to grab fastpasses when you go as well!

Weekdays will by far be less crowded unless they are a holiday. There's actually a very helpful crowd calendar here: http://tdrnavi.jp/forecast/disneysea?lang=en

Please note that while it is very helpful to use it, it is no guarantee.

Yes. It may take you a whole day. But it depends on queues, your walking speed, your lunch speed, and which of the rides you'll going for.

Buy the express ticket for speed.

Weekday non-holiday mornings are best.

I recommend all day. I went this year with my friend.
We were supposed to meet in the morning but went a little after 1pm. We wanted to stay longer! Don't forget to watch the fireworks.
Tower of terror and (Journey to center or the earth I think its called) so fun! All the rides are fun!

Do i have to buy disneysea ticket online?
If yes, what will be website to buy the ticket.

No, you don't have to buy online. You can buy at the gates the day you go or at convenience stores around Japan for example. However if you want to buy online, please see this website:

https://reserve.tokyodisneyresort.jp/en/ticket/list/