http://www.balazsboxing.com/thegym/boxingbasics.htm

has a good tutorial on how to perform the left jab.. Also searching this board you'll find a lot more help on this.
Above all, practice.. As you've already read, your left jab is important (one of the most important) so spend a lot of time practicing and practicing again until it feels like you were born doing left jabs.. and then pratice the jab some more.. =) This advice holds true with any punch or techniques you want to master..
Just seek out some good instruction on proper form (you don't want to become an exert at punching with bad form as it may be hard to go back and fix a problem thats been built up over a long period of training) & practice practice practice.