I have read and heard that one can earn and make good money with trading bitcoins.I also want to give it a shot.Everyone says the basic rule of trading is buy low and sell high but I am confused because which ever exchange I try,the buy rate is always higher than sell rate.How can one profit if sell rate is less than buy rate.If I buy at $390, the sell rate is $380
Well in trading mate thats normal the one that youre saying buy low sell high you should promote the more buyer the the sell price will rise up and you can also wait for long term trading if you have the patience to wait like me im waiting for long term trading all of my coins are sleeping lmao. Goodluck mate 
And in the beginning you have to start with some small amounts and later you can make some more profit.