For an overview, checkout the docs and if you want full instructions on installation - click here for the setup page. After you've got the script installed, then we have things to checkout and advice on getting a full site up and running.
Obviously for a site to be worth looking at, it has to have content! The first step should be to create Categories. It's up to you, but largely we focus on general grouping. Such as 'Blogs', 'Reviews', 'News', etc. If you want to classify items further (such as by game system or by genre), you can use tags to get a paginated list of articles that way.
After categories have been created, you should add some custom fields. This is another area you should think over first - just to try and get a general outline of how content will be added. Some people prefer keeping it simple - having a big textarea where staff will simply enter in the contents of their article, others want to break it down so data can be shown in multiple areas on the page - such as choosing a game system, rather than it just being in the review.
You should be set to start adding content! We also advise to create a few basic pages - such as an About Page and Contact Us Page. This gives a little background on the site and a way for visitors to get a hold of staff.
If you are bringing an existing theme into AdaptCMS, then one of the first things that should be done is creating a theme. After you create it and assign it as the default one, you can upload/add assets. You'll want to add in your css mainly, then add a 'Default' template in the Layouts folder for the new theme. For an example, you can copy over/look at the layout for the 'Default' theme.
There are a few things that must be kept - jQuery, Bootstrap, Global JS file, AutoLoad lines, Bootstrap CSS. You also must keep the fetch content code and preferably the session flash. This might sound like a decent amount, but it really isn't. For the freontend you could skip all of this besides the fetch content code, jQuery and jQuery validate.
Remember, from the documentation, templates for a new theme overwrite. So if you only want to add a layout file - that's fine, all other templates will be then used from the 'Default' theme. This works out great because, usually, there will be no need to adjust admin templates so you can focus on what needs adjusting for your new theme.