It depends on whether the one you open is Nominees Account or CDS Account under your own name.
If you open your account at Dataran Maybank (Bangsar) then most likely your one is not a nominees account.

Nominees account you have to deposit money into the trust account first before you allowed to buy anything. When you sell, the money also will go into your nominees account. You cannot withdraw money from nominess account into your current / savings account using Maybank2u. Must sign a complete a form and send to them via fax or by hand. Usually take 1-2days for the fund to go ito your account. But you enjoy lower brokerage compare to notmal account under your own name.
CDS under your own name, you don't needto bank in money first before start trading. You are allowed to buy first and settle on T+3 up to the trading limit granted to you when you open the account. They will give you an account number to bank in money for your purchase. IF you use Mybank2u to make payment they will charge RM1 transaction fees. When you sell the proceeds will be bank into your maybank account if you have one. This type of account you must open at Dataran Maybank.The brokerage is slightly higher then nominees account.

You are allowed to trade 3 times the money you deposit into your Premier 1 a/c. So if you put in 2k, you can buy shares up to 6k.
Let say after you bank in 2k, you purchase share worth 2k and your Premier 1 a/c become zero, you still can buy another 4k of share without deposting again into the Premier 1 a/c.

Yes it will deduct directly from your Premier 1 Account.  If you purchase amount is more than what is available in your Premier 1 account,you need to deposit extra money into ypur Premier 1 account by T3.