There are several levels of answers to your question, depending upon whether you open a straight account or a margin account and whether or not you will be depositing marginable stocks into the account once it is opened.

As far as what you need to have in your account to do a stock trade, assuming no deposited shares in the margin side, you will need the amount of the purchase you wish to make, plus their standard trade fee plus a few cents extra for their misc fees, assigned by their clearing house and beyond their control to regulate. And it will be a T+3 timing date, if there are no funds available for trading.

T+3 means Trade date + 3days for clearing. So, if you are making a deposit inorder to trade, your T=Deposit date funds are received at TDA, PLUS 3 days before those funds are available to trade with.

Trust this helps.

I'm thinking about switching over to TDAmeritrade and was wondering how much do you need to have in your account to stock trade? Is there a minimum requirement that has to be in the account at all times?