poloniex
Some tips for noobs :
Posting an order is FREE. You can cancel, and post again without penalty.
0.15% fee is applied if transaction happened, and you were maker - meaning you posted an offer, and your peer bought or sold your offer.
0.25% fee is if you are taker, meaning roles are reversed, you took available offer from market, reducing its liquidity de facto.
So both parties pay fee, one pays 0.15% other 0.25%, make sure you pay less, it matters in the long run.
For example if you are trading alt coin with 50k satoshi price range, maker fee will be 70 satoshi (per unit). So you need at least 140 satoshi markup just to pay for fees (you'll have 2 transactions), all above is your profit.
Trading is negative sum game, don't overestimate yourself. Always consider that big players are manipulating the market and have sure profits, leaving average joes in ever bigger trouble. This is not easy money.