Hi i am new to this and trying to learn how to do this right.
I want to start day trading bitcoins and others but I see many problems or things I do not understand so i have a few ?s

Lets start with using say using poloniex I am in the US so am limited in what trading sites I can use.

With poloniex they want me to add money into trade with by adding in Bitcoins that just seems nuts to me for example.

Let's say I added in Bitcoin that is at say a value of $2700 I use that to then buy other coins and it see a gain of say $100 so i sell the other coins back to bitcoin but in the same amount of time Bitcoin drops to $2600 so after trading fees I have now lost money do to the price of the Bitcoin going up and down over the day.

I would like to trade and buy with US money so lets say I use US money and buy a $2600 bitcoin and it then jumps up to $2700 and I sell it back to US money so i now have $2700 - any fees making a profit as the US money is not going up and down
I see no way to do this with the poloniex trading system.

Am I missing something here with Bitcoin going up and down so much i would not want to use it as the base money to trade with?

It looks like after i ever get approved a few other sites will let me buy in with US money and then buy and trade Bitcoins but they only trade Bitcoins and one other coin so i am left out of trading all the others I want to be able to trade with as I want.

Thanks for any insight on this.