I am very new with Bitcoin and I dont know how to trade or purchase bitcoins at a Bitcoin exchange?
You need to have an online wallet that enables you to buy bitcoins.
Trading is so simple. You buy a bitcoin, if the price is lower than the usual then sell your bitcoin if the current selling price is way higher than the buying price, the time you purchased your bitcoin.
Basically, you also need to consider the processing fee of your chosen online wallet and the time of processing.