How to Install phpBB
The installation process of phpBB is pretty similar on all control panels, but I'm assuming you have purchased hosting that uses CPanel. CPanel is the most common control panel in the web hosting industry, which is why I chose to make my installation guide based on that. Plus, I use web hosting with CPanel so it makes things a tad bit easier on me :D.
Softaculous
If you have CPanel, then you most likely have Softaculous. Softaculous is an auto installer that allows you to install almost every popular free script in just a few clicks. To see if you have it, log into your web hosting CPanel account and search for "Softaculous". If you have it, then click on it.
You will probably see phpBB under the "Top Scripts," but if you don't then you can find it under "Forums". Hover on "phpBB" and click "Install"!
Everything is pretty much explained on the installation page.
Regular Install
1.) Download and extract phpBB.
2.) Upload the extracted files to a web accessible directory on your web hosting account. (Public_Html, or a sub-directory in the public_html)
3.) Use your browser to go to the location where you uploaded the files with the addition of install/.
For example, example.com/install/ or example.com/forum/install/
Click on the "install" tab, then click on "proceed to next step" and then click on "start install".
4.) Create a database by logging into CPanel, and by going to the "MySQL® Database Wizard".
Enter a name in the "New Database" field and click next.
Enter a username, a password (make sure to remember it) and click "create user".
Check "All Privileges" and click "next step".
5.) Go back to the phpBB install page and enter the database and username information.
Ex:
Click "proceed to next step".
Your test connection should be successful.
6.) Setup an admin account by filling out the admin configuration fields.
Ex:
Click "proceed to next step".
You should pass the administrator settings test.
Click "proceed to next step".
7.) Click "proceed to next step" on the "configuration file" page.
8.) Click "proceed to next step" on the "advanced settings" page.
9.) Click "proceed to next step" on the "create database tables" page
10.) Delete the phpBB3 install directory off of your webhost.
After you've done that your phpBB forum installation is complete. All you have to do now is login and edit your board settings to change your site name. In addition to changing some default settings, you will want to install a theme!
The theme is now installed and all members will see it by default.
Conclusion
Well, that's about it. You've now installed phpBB and your first theme! Anyway, I hope your installation went swiftly and if you have any questions, then ask in the comments below!


How to Install a phpBB Theme
1.) Download and extract the theme. 2.) Use an ftp program to move the extracted theme into the styles folder of your phpBB3 forum. 3.) Now you can set about installing the newly uploaded style. Go to your forum, login and go to the ACP. Once you're in the ACP, you need to go to the “styles” tab”. In the styles tab you should see the style that you just uploaded, all you have to do is click "install", check “make default” and click "submit".