I tried to start my 2001 VW Jetta, the lights on the dash flickered off and on. Replaced the battery now it does nothing. ideas on why?

Make sure the battery isn't in backwards. Red wire to + on battery. Make sure there is no corrosion at the terminals and they are good and tight.

ooooo...gonna get in trouble for suggesting this as this comes up on EVERY car in the wet or salt belt states...and in humid environments too...what happens is the Black 1/0awg Cable to the chassis ground/fender quickly gets corroded with dis-similar metals exchanging "Melt" temperatures of 300 amps...this corrosion creates a "semi-conductor" situation and corrodes with salt spray and water...and makes the power NOT reaching the ground....SO take a wirebrush...undo the lug that connects to the fender/chassis ground and wirebrush the lug (both sides, use a pliers to hold it) and the territory where the lug bolts to....scrub it till shiny to allow the amperage to get thru....bolt it all together for an ignition system that will see the voltage...the headlights will be brighter and the ECU will stop throwing up trouble codes because the 02 sensor cannot heat up with only 10.5 volts and throws a trouble code...this will all disppear when you do a bit of tidying up on the system ground~...thanks~

the small battery terminal=NEGATIVE /////Biggie=Positive....practically impossible to get wrong....with aftermarket lugs is EASY to screw up tho~

Excellent advice fellows I greatly appreciate it,and will be back tomorrow with my results.

If someone did get them backwards...you will have to replace the MEGA fuse and/or fuseable links ...from the battery cable TO the chassis is one of them...alternator may have one as well...these are to burn up and stop FIRES from happening...stinky and will alarm you if you're cookin'~

I am having the same issues with my '01 Jetta. The battery is good and the car always begins to turn over, but it never completely starts. As soon as I let go of the key, the car shuts off.