I have never been to a Buddhist 'church.' The only time I have ever heard a Buddhist place of worship called a church is with Shin Buddhism, which is not a branch of Buddhism I am familiar with. So, I will be interested in hearing about your experience at a Buddhist church.
In my experiences with various Buddhist temples and centers (predominantly Zen, Tibetan and Shambhala Buddhism), you spend the first 15-30 minutes meditating then have a dharma talk for the remaining time. Additionally, beginning meditation classes are offered once or twice a week. Meditation (and there are various types of meditation) is the key practice in Buddhism.
As for what to be aware of, I am not sure about a Buddhist church. I would probably wear something comfortable, as I imagine you will be doing some meditation. And, I always err on the side of conservative when going to any church/temple/mosque/religious center.
Have fun and please share your experience!
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