First things first, thank you, Shalita for asking questions that lead to this post.
Did you know that when you installed WordPress through your hosting service, WordPress set up a default theme, either the Twenty Ten or Twenty Eleven Themes presently, both are a wonderful platform to begin with, and either theme provides you with the tools needed to make changes, all or most WP themes include a way to upload new themes, plugins, photos and media files from the dashboard of you new WordPress website.
With that in mind, let me facilitate your progress by letting you know that whether you have a Mac or PC does not matter where WordPress is concerned unless you are self hosting your installation on that Mac or PC, and then it does matter. So, because you are uploading data to another site means that you are not self hosting on your local machine, [which again, some folks do for testing purposes] and that is important for this reason; you need only to download themes and plugins to your home machine, and into a single folder on that computer so that you can find them easily under pressure. They should be in a zip format. It does not matter where that WordPress compatible software is downloaded from as long as it is clean [no Trojans or viruses] and is in a zip file.
Download several Themes [I have perhaps 10 installed but inactive at present] and Plugins [I have perhaps 20+ plugins installed and half are inactive] and later when you upload and install them, do not activate them immediately. Preview a theme and decide if will fit into your scheme of things, but before you commit to a installing a new theme, remember, many of your settings and tweaks will have to be repeated with every new theme that you activate. The same is true for plugins; try them one at a time and you will in time decide on a core group of plugins that enhance your themes-some of them will be a real pain. The keepers you choose will be a matter of personal preference and situation. Note: I will provide a short list of themes and plugins that I have installed available at the end of this post.
We established that WordPress has a built-in file server that can upload [and download] your zip files. Here is how you use it. To add a theme to your collection, you access this file server from your dashboard panel by sliding your mouse to the Appearance button and then to your Themes button, clicking on that Themes button opens up your Manage Themes page; clicking the grayed Install Themes button opens another page, on the upper top left under the grayed Manage Themes button you will see a light blue Upload button, click on that button to open a new page and there will be a Install a theme in .zip format search box, click on the Browse button, it should open up your home computer files and allow you to open the file where you have your themes stored, click on the theme zip file you want to upload and its information will appear in the search window; click Install Now. Go ahead and upload several that interest you.
Note that if the file is a copy of one that you installed already, it will not install it again and will let you know that fact, and if you try to install a plugin zip file into themes or vise versa, it will not let you do it. The theme designers built-in a few safeguards to protect us from ourselves for those occasions when we are operating under pressure.
Uploading Plugins is essentially the same method as with the themes. From your dashboard panel slide your mouse to the Plugins button and then to your Add New button, clicking on that Add New button opens up your Install Plugins page; clicking the grayed Install Plugins button opens another page, on the upper top left under the Install Plugins heading, you will see a light blue Upload button, click on that button to open a new page and there will be a Install a Plugin in .zip format search box, click on the Browse button, it should open up your home computer files and allow you to open the file where you have your plugins stored, click on the plugin zip file you want to upload and its information will appear in the search window; click Install Now.
You can upload Media following the same steps as indicated for the Themes and Plugins for general use, or you can do it when you are posting, there is an upload link on the top left of the text box that give you the option to install photos into that specific post, but not into the general library. The media library option vs the specific post option is a matter of personal choice and both are easy to use.
A short list of theme zip files that I uploaded and installed but are not necessarily activated, those marked with as * XYZ theme are activated:
• Twenty Ten theme
• *Twenty Eleven theme
• Twenty Twelve theme
• *Blogs MU theme [use with Buddy Press 1.6.1 for expanded features] the child theme is buried within theme documents
• Thematic theme
• 30398_Magazine theme
• 30398_Scholar theme
• 30398_Showcase theme
• WP-portal theme
• WordPress 3.4.2 theme [for back up files]
• Notes-mag theme [a sandbox project theme]
• WP-framework theme [a sandbox project theme]
• And others
A short list of plugin zip files that I uploaded and installed but are not necessarily activated, those marked with as * XYZ plugins are activated:
• 30398_floating social
• *Buddypress 1.6.1
• Php snippets
• *Ada WPMS Sitewide Feed
• Jetpack by Wordpress.com
• Page Links To
• *WPMU Site Maintenance Mode
• And many more
All of these files can be uploaded using Filezilla Client or other FTP tools, but it is much easier to use the built in WordPress tools. Get familiar with them . . . they do dot bite or require potty training.
Having your own domain and website(s) should never be grueling or a nightmare to manage, it can be if this is unfamiliar ground, I hope this helps clarify things.