Consider starting with the free Su-25T. Search for tutorials on youtube, there are plenty, and see how you like it. If you're a hardcore simmer, you'll want more (clickable cockpits). In that case, I cannot recommend the A-10C enough, it's currently the most complete simulation of a modern military jet. The MiG-21 is also a great choice.
If you like WWII stuff, then try out the TF-51 included for free in DCS World. It is unarmed, but will give you a taste of aircraft from that era and also of full fidelity DCS modules. These WWII aircraft are not so misplaced anymore, given that a large project involving a map and various aircraft and units is planned to support second world war scenarios.
About FC3, if you buy it, you'll have the exact same modules as Su-25 for DCS World, A-10A for DCS World, F-15C for DCS World, Su-27 for DCS World, and a few extras, such as Su-33 (6DOF cockpit but no professional flight model), and some MiG-29s that come straight from flaming cliffs 2. If you prefer to buy separate, I recommend Su-27 as it comes with a full set of interactive training missions.
Hope this was of help, good flights!
