SimplePress and MingleForum's website have downloadable themes. They come with default themes that are separate from whatever theme you use on WordPress.
To install plugins there are different ways to going about it.
1. In the WordPress Dashboard, go to Plugins and click "Add New", you can then search for a plugin and click install under its name.
2. You can download a zip file of a plugin and upload the contents to your website in the wp-content/plugins folder, then activate it in your Dashboard > Plugins
3. You can download the zip file for the plugin, go to your Dashboard > Plugins > Add New and use the upload option which will allow you to choose the zip file on your computer and it will upload it and unzip it to the proper location.
As far as managing the forums, each one is different and SimplePress is more complex than Mingle. Check out their documentation for more detailed instructions. I suggest to get your feet wet, use Mingle first as it is more simple, you can always remove it and use a different forum.