Picking the wrong broker
Although brokers all seem to offer the same product check the fine detail. It’s essential any profits you make are not swallowed up by choosing a broker with wide spreads, excessive fees, and poor swap rates.
Also consider the payment choices as 3rd party bank fees etc. all erode your precious profit especially if you are trading with a small account.
Also ensure that if they go bust (Alpari anyone?) you are compensated. (It happened to me in 2015, I got my £20k back through the FCA compensation scheme here in the UK so it does work - although my GBP funds were converted into USD then paid to me in GBP so I lost a fair bit on the currency exchange, oh the irony!)