PRACTICE, PRACTICE, PRACTICE
This can be done in both the open water and your local swimming pool.
Practice in open water as often as you could to familiarize yourself with swimming in that setting. Find open water communities, clubs, coaches, experts where you could learn open-water specific skills such as sighting and drafting.
With your training buddy, team, friends, go for open water swims as often you can but always put safety first, never swim alone.
To get better in open water swims you should practice in pools as well. Consistent training in the pool, using a mixture of drills and internal-based swim sets, will allow you to become as comfortable and efficient as possible in the open water, helping you cope with anxiety.
Video analysis is also very helpful tool to improve your open water skills.