What is Metal Gear?
Quote: Wikipedia (for now)
Metal Gear (Japanese: メタルギア Hepburn: Metaru Gia?) is a series of action-adventure stealth video games, created by Hideo Kojima and developed and published by Konami. The first game, Metal Gear, was released in 1987 for the MSX2. The player takes control of a special forces operative Solid Snake who is assigned to find the titular superweapon "Metal Gear", a bipedal walking tank with the ability to launch nuclear weapons. Several sequels have been released for multiple consoles after requests from Konami to produce new Metal Gear games. The sequels expand the original game's plot adding new characters opposing and supporting Snake, while there have also been a few prequels exploring the origins of the Metal Gear and recurring characters. Various parts were inspired by Hollywood films with character's names, settings and artworks often referencing them.
How do I get into Metal Gear?
It's best to play the games in order of release, however the original MSX games are skippable and you can wait until later to play them, they are pretty short anyway. You could buy the HD or Legacy collections to do this, but the HD collection does not include MGS1 so you may want to buy it separately, it's available on PC, PS1, Gamecube (as a remake, it's a little... weird compared to the original though) and is on PSN too. If you cannot play MGS1 for any reason then MGS2 is fine to start with, it's available on Xbox, PS2, Xbox 360 (through the HD collection), PS3 (through either Legacy or HD collections) and PC (however the PC port is a bet of a mess and will require lots of fan made patches to work on modern systems).
Note- Do not post Phantom Pain spoilers until the game is properly released, even then you MUST use the appropriate spoiler tags.