Worrying too much about swim technique:
Get the basic technique in place from the start then start working on building your base and getting swim fit. A poor swimmer needs 4 sessions per week minimum.
Limit the number of training tools you think you need to buy:
Paddles, kickboards, fins, snorkels etc are all great to use but it comes at a price. Swimming faster and getting fitter doesn't necessarily mean you have to buy everything that is available. You can achieve the same in training with less expensive resources.
Too much too soon:
If you start out new to swim training and push yourself too hard, too far, too soon, you do risk injuries, particularly in the shoulder and arms. First, build the base then up the mileage and intensity.
Starting too fast:
This is a natural reaction because your adrenaline juices are flowing at maximum level. Start out a little slower over the first couple of hundred meters and you will end up with a faster swim time.
Swimming with hundreds of others all clambering over one another has seen the very best of athletes lose their cool and begin to hyperventilate. Roll onto your back, take off the goggles and take a few seconds to regain composure mentally and you will be okay to carry on.
Swimming off course:
You need to find your own markers out in the open water and not rely on others to point you in the right direction. Make sure you know where you need to swim before you start out. There's no sense in wasting valuable time and swimming extra distance for no reason.