Go into your Module Manager, which is located within your "Extensions" drop-down. Here you'll see a list of pre-existing modules. If you're working with a fresh installation of Joomla, your list might be very short, or even empty by default, depending on if your Joomla included the demo content or not.
At the top of the page, click "New."
Here you'll see all the individual modules you can choose to install. A lot will depend on what plugins and extensions you've installed. For example, everyone will have access to Poll, Random Image, Custom HTML, Banner, Footer, etc etc. However, I had to purchase and install both the RocketTheme and YOOtheme extensions, so these will not be available to the beginner user. Here I'll use YOOtweet to just quickly show how you can display a Twitter feed on a Joomla site.
Fill in whatever parameters your extension asks before you do anything else. YOOtweet asks for one of a few different options, such as tweets referencing a user, containing words, containing hashtags, or from specific users.
Next comes the details - Joomla asks you to imput a few specific things before you can show it. For example, it asks for a title of a module, so it can be identified later. I'll just name mine "Twitter." It'll also ask other things of you, like if you would like the title of the module to be displayed. I personally like this option for plugins that might not be clear, such as Twitter extensions, login extensions and the such. For images, banners, and the like, this is better left off. Be sure to make sure "Enabled" is checked if you would like your module to be displayed.
Joomla also asks for positioning. You should have a vague idea of how your template works before you change this. Most columns are at the very least three column layouts, so placing your work in either the left or right column is always a viable option. All Joomla templates also include a footer option as well. the drop-down column wil provide you with all the possible positions for your specific template, but keep in mind that just because you can put something in that specific area, it doesn't mean that it'll look great there. But luckily it is very easy to move it to a new position, so feel free to experiment a little! For the sake of ease, I'm simply going to add it to the right column.
The option "Order" tells Joomla where to place your image in the event that there is more than one module in that specific column. The higher the number, the lower on the list it will appear. The access level attribute allows you to allow modules to be only visible to certain people. Here, I'll leave mine as "Public" which allows everyone on my site to see my tweets.
Menu Assignment simply states on which pages your module will be visible. Normally, you're going to want it to be visible on all of them, so I'm going to leave the menu option "all" checked.