I'm Brand New to Recording in General

I'll second ryclark's suggestion of reading/watching some tutorials, but if you want the "Quick Start" guide;

Connect the 18i20 to your computer and install the drivers provided by Focusrite.  Don't forget to got to the Focusrite web site and download the latest  versions...the ones that come in the box are usually old.

Open Audition.  Go to Edit/Preferences/Audio Hardware.  Under "Device Class" select ASIO and on the drop down menu for "Device" select the 18i20.  It may be your only choice or there might be others depending on what you have in your computer.

Okay and close the preferences box.  Click File/New/Multitrack Session.  Give it a name and select where on your computer you want it stored--usually its a good idea to to create a new folder for each project/session.

Once this is done, make sure you're in Multitrack View (there's a selection for Multitrack or Waveform top left of your screen.  In Multitrack you'll see a bunch of empty tracks.  At the left end of each track is an edit box where you can select what inputs are used, playback levels, that sort of thing.  Click on the input/source box--it's a drop down menu probably showing something like "Default stereo input" to start with.  Click on the menu and select Mono then on the list that will pop up, pick the track you want from the 18i20.  Repeat this until you have all the 18i20 tracks selected.   If you find you need more individual tracks, go to Multitrack/Track then "Add Mono Audio Track.

Once you're session is set up, I'd suggest you go to File/Export/Session as template.  this isn't necessary but it will prevent you having to redo all the setting up next time you start a session.

To record, arm each track you want to record in by clicking the litlle letter R in that edit box at the left end of the track.  It'll turn red when you do.  Then, when you actually want to record, hit the red record button on the transport controls.  Note that recording levels are set with the knobs on the front of your 18i20.  Make sure you set levels so you have lots of headroom.  There's not need--and a lot of disadvantages--to recording every track as loud as you can.

That's recording.

Mixing can be a simple or complex as you like since you can add tons of effects, change levels, group channels into a bus so you can control the sound of the whole drum kit from one track, etc.  For this I really do suggest going through the tutorials.  However, at the most basic, I suggest you have a look at the use of Volume Envelopes which are an easy way to automate your levels:  Automate mixes with envelopes in Adobe Audition CC

Beyond that, there's a good chance you will want some effects like dynamics processing and EQ--a quick search will bring up more info than I can type here!

Have fun experimenting!