I agree with Pull. One mistake I made (in addition to all of the ones Pull mentions) was listening to other people who knew nothing about lifting. The worst thing is when a group of guys who all have terrible routines/training/ect and they all reinforce eachothers bad habits, then said people start giving advice to newer guys. The newer guys dont know any better and listen.
Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure.
"When in doubt, just get really, really strong. It tends to cure most problems in training – and life." - Wendler