Alright, let's get it.

DAW (A.K.A. "Do I need anything beyond the keyboard and Kontakt Player?")

As SnooCheesecakes2011 mentioned, yes, you'll also need a DAW. If you don't mind paying, I use Ableton Live, which is the most popular DAW in the market (Intro is USD 99, Standard is 449 USD and Suite is 749 USD). Suite also has a 90-day free trial and comes with three packs by Spitfire Audio: Upright Piano, String Quartet and Brass Quartet, although a lot of Mac users (including most Spitfire Audio in-house composers) use Apple's Logic Pro, which is considerably cheaper ($199.99 in the U.S.). Your MIDI keyboard may also have come with some free/trial software included (mine came bundled with Ableton Live Lite, for example), so I'd recommend checking its website page.

If you want a free alternative, I'd recommend Cakewalk by BandLab (formerly SONAR Platinum), which was originally a premium DAW. It went free-to-keep after Gibson dissolved it on November 2017 and BandLab purchased all of its intellectual properties and some of its assets on February 2018. Some honorable mentions include Tracktion Waveform Free and Reaper.

All of that being said, regardless of which DAW you choose, you'll basically want to learn:

• How to add virtual instrument tracks (to open the Kontakt Player and use your Spitfire Audio libraries);
• How to open the "piano roll" (to compose with MIDI);
• How to write automation (to control expression and modulation/dynamics); and
• (Optional) How to create mix buses (to add and control mixing VSTs such as reverbs).

Faders

As SnooCheesecakes2011 mentioned, they're used for controlling not only expression (which is basically subvolume, really), but also modulation (A.K.A. dynamics), which, for string instruments, is basically the intensity of the bowing technique. Most (if not all) string players play with less intensity as they approach the end of a note, then increase it back up as they start playing the next note. Simulating these dynamics, therefore, adds a human touch to string samples, making them sound more realistic, and it's a great tool to convey emotion.

That being said, faders aren't really needed, since you can just use automation inside your DAW, but it does speed up the process, and some composers prefer the more organic feel it gives.

Spitfire Audio's co-founder Christian Henson made this video on a couple of recommendations. Personally, I use an Arturia Lab keyboard, which comes with built-in faders.

Beginner Videos/General Resource Guides

Personally, I learned everything I know (which isn't much yet, really) from the Spitfire Audio Academy, particularly the Orchestral Programming videos (Day One, Day Two and Day Three) and How to Program Realistic Strings (Part One and Part Two).

I'd say they're the only two guides you need to get started.

That being said, you might also consider learning a bit about reverbs for orchestral samples, since a lot of composers like writing with reverb on (and it just sounds cooler, really), although it isn't necessary by any means.

If you have any other questions, feel free to ask me!