What will happen after the segwit in btc and altcoins in any trading sites? What will be its possible effect to all traders?
Segwit is only going to activate on bitcoin not all coins. It shouldn't be misunderstand with Aug 1 events. Segwit ain't going to come now, the possible segwit fork will be coming in few months. Currently you need not worry much about exchanges. They will be close for one or two days to see to bcc fork after that market will regain its momentum. As trader, you can be assured that prices of coins not going to change much.
Firstly happening of this transition is pretty low as people are supporting traditional BTC and it won't go down so easily. It's like second split is a joke and devs must be finding a place to hide themselves. Lols.
Though it happens it will affect bitcoin price and trading, investing, exchanges will be shut ultimately leading to dropped price as there won't be any "theoretical" volume of money transacted in bitcoin. Other coins are completely different story and they won't be having "direct" effect but will get affected as the market of bitcoin fluctuates.