You can now find the serverlist at http://servers.simutrans.org/
. There you can see the list of online and recently online servers sorted by what base pakset they use. By clicking the "Show detailed information" you can see what version of the Simutrans executable and pakset that you need to join a particular server. Many servers have an info page on this section of the Simutrans forum, which you should read before playing.
Most servers today use the latest stable release, which you can download here
. Other servers may use a nightly version of Simutrans and/or the pakset, which you can download from http://nightly.simutrans-germany.com/
. Note that some servers using a nightly version of the program use the latest stable version of the pakset. Unzip the correct version of the program and pakset, and place the pakset inside the program directory.
When you start simutrans, press "Play Online" in the splash screen, or press "n." This will bring you to the server selection dialog that will display all online servers running your version of Simutrans and your pakset (note that if the pakset contains addons that you don't have, it may still display). Click the server you want to join, enter an optional nickname, and press "Play Online." If you are missing addons, that button will be greyed out. If you don't see any servers, check to make sure you have the correct version of the pakset and that the server you want to play is online.
When you join the server, you will be placed in the first player slot, which is usually locked. You will need to create a new player: do this by opening the players menu (options>Players), finding an open slot (grey, "slot empty" with no player name), selecting "Manual (human)" and then pressing the checkbox to the left. Then click the small arrow to switch yourself to that player.
Chat is in the top bar of the message center.