Hi Racal, welcome to the pound!
I'm the resident DCS acolyte round these parts, having been waving the flag for the sim since the A-10C first arrived. I have long been trying to lure the Dogz into DCS's dark and tortuous systems through it's seductive graphics and sense of realism. I have managed to ensnare a few unfortunate souls en route but more victims enthusiasts are most welcome.
I have a reasonable knowledge of most (though not all, I hasten to add) of the modules available so am happy to help with any you choose; or I can at least point you to another DD or some online content that may be more knowledgeable or applicable to the module of your choosing.
As to where to start? This is always a difficult question. Each module has it's plus points and limitations. For example: the F-14 Tomcat is an awesome machine for A2A and in some regard for bombing but is not so friendly to neophyte jet users with edge of the envelope handling that can be tricky and the lack of a HUD that some jet users rely on to provide feedback. Then the Spitfire; it is a fabulous dogfighter but a pain to get in the air or back on the ground until you've got a lot of practice in. On top of this it is the slowest of the late-WW2 era aircraft so careless use of the stick can leave you stuck grinding holes in the sky as your enemies Boom and Zoom around you. The A-10C is an incredible payload carrying death machine to ground forces, yet it's slow and requires a lot of practice and learning to employ the many systems - just getting an AGM-65 Maverick off the rail can require a 10-step process if you're using other sensors to find targets. The F/A-18C is an amazing all rounder but with slightly under-powered and slow spooling engines can be tricky to carrier land and as the pilot you're always worrying about fuel as it' s at something of a premium in the Hornet.
In short, all of the modules have a capacity to frustrate or provide a significant hurdle that requires some measure of stoicism or sheer bloody mindedness to overcome. However, the sense of reward is commensurately edifying when you do finally nail it.
Because of this it is best to choose an airframe you already have some emotional investment in - something you have admired since child-hood, or have been seduced by the appearance or mystique of before you even discovered DCS, as this will tend to drive your motivation to overcome the challenges inherent to your chosen module.
Choose a module that calls to you.
I can't recommend a single module, because all of them are incredible on some level!