The Users and Developers Guide on the wiki (
http://wiki.minix3.org/) should have enough information to get you started.
Download the latest ISO development snapshot (which is 3.4.0rc5 as of now) from the downloads page and install it inside a virtual machine. You can then install packages, run X11 or do other UNIX things. Since MINIX 3 has integrated the NetBSD userland, please refer to the NetBSD documentation too for additional information.
If you want to start developing, you can cross-compile MINIX 3 and start coding (instructions are on the wiki). You can also contribute documentation, test pkgsrc packages or just report bugs if you wish.