There are many varieties of Buddhist gatherings and centers. Some are almost entirely devoted to silent meditation and others are more like what most Westerners think of as worship services, with a sermon by a priest, monk, or nun, and announcements by a leader of the temple.
Typical components of a Buddhist congregational gathering would be chanting, an incense offering, silent meditation, and a sermon or talk. It can last one or two hours.
You should wear loose, comfortable, causal clothing to a meditation center. SOME (few) temples expect women to wear a dress or shirt and blouse. Call the center to find out about that.
No head covering is required, nor are there any color restrictions for clothing.
Most Buddhist temples will have an altar (with a statue of Buddha), side altars (with statues or pictures of the founder of the particular lineage of Buddhism followed by that temple), and either pews or meditation pillows/cushions.
You should arrive early. Where you sit depends on the tradition of the temple. If there is a seated meditation, you'll be directed to a cushion. Ask, if you don't know.
If you arrive after the service has begun, don't enter during meditation. Don't leave during meditation.
It's entirely optional for you, as a visitor, to chant with the congregation, or to stand when the others do. There is NO part of the goings-on in which you may not participate.
Do not take photos or videos, do not use a tape recorder, unless you have prior permission to do so.
In some temples and meditation centers there may be an offertory box near the front of the temple. It is customary to contribute between $1.00 and $5.00.
There may or may not be a reception after the service, in a separate area, at which light (and generally vegetarian) food may be served. This reception (if there is one) can last up to an hour. it is NOT considered impolite to not eat or drink. Some temples have a form of blessing said before or after eating or drinking. Take your cues from the others - if people seem to be waiting for something to happen before they eat or drink, do likewise; there will probably be a blessing said.
It is FINE to not participate in rituals or meditations if you're uncomfortable about them. If you choose not to participate, you should sit quietly and still. In general, simply do NOT talk during service.
Have a good time!