"Day trading typically only applies to margin accounts. Due to Federal Reserve Board (FRB) regulations, it is extremely difficult to day trade in a cash account. In a cash account, both the buy and the sell have a three day settlement period. If you sell a stock before the three day settlement period on the buy is up, the proceeds from the sale are on hold until the buy has completed its three day settlement. For example, if you buy a stock on Monday, and sell it Monday, the funds are on hold until Thursday. These regulations limit the ability to day trade in a cash account.

The National Association of Securities Dealers (NASD) currently defines day trading as buying and selling, or short selling and buying to cover the same security on the same day. NASD Rule 2520 defines a "pattern day trader" to include clients who day trade four or more times in a five-business-day period.

Pattern Day Trading Accounts

This rule requires accounts identified as participating in "pattern day trading" to maintain at least $25,000 in equity. These accounts may be given increased buying power up to four times their maintenance margin excess for day trading purchases.

If the equity in the account drops below $25,000, a day trading minimum equity call will be issued. Total purchases will be restricted to two times SRO excess, based on total trading commitment, until the equity in your account increases to $25,000 or more.

my friend said that i have to wait three days OR i can sell it the same day though.

Day trading is the practice of opening and closing an equity or option position (i.e., buying and selling the same stock or option) on the same trading day. A same-day buy and sell, or a same-day sell short and buy to cover, is considered a day trade.

If you make four or more day trades in a rolling five-trading-day period, you will be considered a pattern day trader under these rules. However, if the day trading activity does not exceed six percent of your total trading activity for the five-day period in question, your account may not be designated a pattern day trading account.

If you meet the definition of a pattern day trader, you'll be required to maintain $25,000 equity in your brokerage account at all times. If your account equity falls below $25,000, a day trading minimum equity call will be issued on your account requiring you to deposit additional funds or securities.

As a pattern day trader, you'll be granted day trading buying power up to four times your NYSE margin excess.

Note:
Per NYSE Rule 431, stock transactions in cash and retirement accounts are subject to T+3 settlement. In other words, if you sell shares of stock held in your account, you may not use the proceeds to purchase another security until three market days after the trade (the settlement date of the sale). Options trades are subject to T+1 settlement.