How do people who practice baha'i worship?

do they meet in a church? do they sing? What happens at a weekly meeting?

For the answers to all these and more, consult your local Wikipedia.

They built splendid gardens in Haiffa.

They meet in someones house and read sacred writings of different faiths, listen to things Baha'allulah wrote, have some music. In each continent, they have a lovely building too but it's open to anyone to worship there - in Australia, it's in Sydney, in Asia it's in New Delhi. Their main cent re is in Haifa in Israel - the Universal House of Justice.

There are no Baha'i churches per SE. Baha'is generally meet in believers' homes or in community-owned properties; larger communities may rent out meeting halls (sometimes, but not usually, including churches--I have heard colloquially that Universalist Unitarian churches are particularly open to permitting Baha'is to use their facilities). All that being said, there exist Baha'i "Houses of Worship" which are analogous to large temples. Currently there are only seven (Chicago, Panama, Samoa, Germany, Sydney Australia, Uganda, and New Dehli India--forgot that one the first time!) with one in planning/construction stages in Chile, and one in Turkmenistan which was destroyed in the mid-20Th century. Houses of Worship are different from ones traditional idea of a church, however, as (a) they are open to people of all faiths, (b) sermons are not held there, as the only words permitted to be spoken aloud in them are holy scriptures (from any faith). Ultimately the Faith's goal is to have a House of Worship in every community, but obviously that has yet to occur

Worship in the Baha'i Faith is very non-ritualized and often reflects the local culture of the community. Baha'is hold regular meetings called "Feasts" every 19 days, reflecting the Baha'i calendar which is 19 months of 19 days each (with a few left over). At these meetings there is a business portion, a social portion, and a worship portion. The worship portion can be free-form; reading of holy scripture (of any faith) is performed, prayers are said, and song (both recorded music and spontaneously sung/played by participants) is very common. The business portion is where correspondence is read/discussed and community issues/concerns are discussed. Generally only Baha'is attend Feasts.

Outside of Feasts, Baha'i communities often have devotional meetings, "firesides" (educational/devotional gatherings), study sessions, and so forth. These are always open to any person. They occur at a frequency comfortable to the hosts/facilitators, anywhere from weekly to monthly. They are often grass-roots efforts and depend a lot on individual initiative.

Baha'is pray differently than Christians; whereas the only set prayer for Christians (that I'm aware of) is the Lord's Prayer, Baha'is generally recite prayers written by Baha'u'llah, the Bab, and Abdu'l-Baha. There are many prayer books available for this purpose, and prayers cover almost any topic you can think of.

Hope that gives you some good information!