I’m a convert to Catholicism from evangelical Protestantism.
The best way to learn about a Protestant church is to visit that church (make an appointment with the pastor) and talk to him/her, and also ask for a copy of the church’s "Statement of Faith."
This document might have a different name depending on which church you are visiting. I’m guessing that many of the non-denoms have it all on their websites rather than wasting money on a paper document that sits in a rack on the wall.
But basically, this document will describe what the church believes.
The mainlines (older Protestant denominations;, e.g, Presbyterian, Episcopal, Lutheran, Methodist, Congregational, United Church of Christ, Reformed, etc.) will also refer you to their Catechisms and Confessions. Often, these are fairly short documents (not like the Catechism of the Catholic Church :)), so go ahead and look them up and read through them.
The evangelical churches will have a written (or online or both) Statement of Faith that is fairly short.
It’s very important to determine whether the Protestant church you are interested in learning about is “autonomous” or “denominational.” Many of the denominations actually consist of a group of “autonomous” churches, and the denomination exists mainly as a “fellowship organization,” not an authoritarian organization. An example of this is the Evangelical Free Church in America–the “denomination” organization doesn’t have any authority–all the individual EFree churches are autonomous.
In other Protestant denoms, the denomination DOES have authority over ALL the individual churches. Look for an organization within the denomination called “convention” or “consistory” or “council” etc.–these organizations make decisions that are given to the individual churches, and the individual churches are expected to accept and implement these decisions. Also, the individual churches are expected to send a certain amount of money to the denominational headquarters. An example of a Protestant denomination like this is The Christian and MIssionary Alliance–very strong central authority.
And as someone else on the thread already mentioned, be very careful not to assume that all the churches that call themselves a certain name (e.g., Presbyterian) are the same kind of Presbyterians. There are hundreds of different kinds of Baptists, and although they share many common beliefs, especially the Fundamentals of the Faith, they also are very very different.
That’s why it’s good to visit each Protestant church individually and get the Statement of Faith (or whatever) from the pastor of that church. Don’t assume anything. I once ran across a Southern Baptist church that allowed a coven of witches to meet in their building, and the pastor of that SBC was one of the most vocal pro-abortion advocates in the city. But right down the road, there was a Southern Baptist church that led the way in pro-life actitivities in the city!