Almost any stroke will do but front crawl is the most efficient and often the fastest.
You cannot use back stroke as you are likely to be hauled out of the water because generally, lying on your back with your arm in the air is a sign you want help, also for safety reasons you cannot swim backwards because it could be dangerous to you and other competitors around you.
The swim, is it really difficult because you are with lots of other people all with different swimming abilities?
Here you need to try and be a little more confident with your swimming, try to relax, think about your training and try to find space around you in the water, don’t get caught up with the fast swimmers (if you’re not a fast swimmer), move to the back so you can swim at your own pace. Also before you take part in an open water swim try to find where there are open water swimming nights before your triathlon, this will help your confidence no end.
Wearing a wetsuit may feel strange but it can help your swim. Wetsuits help buoyancy and can aid your confidence. It is recommended you try the wetsuit well before the day of the event as they can feel tight and restrict your breathing (don’t be afraid to ask for advice at any time).