I suggest you snag the new Battletech Starter Set, plus 24 plastic 'mechs of various weight classes. BattleTech's a game where you're going to want more than one 'mech; most of the games I've been in you controlled one to four 'mechs, and often more. If you can, look for the newest edition of the starter set, as it's supposed to have much better quality miniatures in it.
After the starter set, you'll want... more 'mechs. And possibly vehicles, aerospace assets, infantry, and power armor. BattleTech releases the fluff and stats for these in sourcebooks called Technical Readouts, which are grouped by year; certain years are "eras". The classic (and only _true_) BattleTech era is the Fourth Succession War, which is covered, IIRC, by the 3039 tech readout. Guardian has a bunch of these, and you can flip through them and find the ones you like. Bear in mind that 'mechs from different eras do not mix terribly well, especially for newer players. The classic bad combination is 3050 Clan Omnimechs vs. vanilla 3025 Inner Sphere Battlemechs, which usually results in an extraordinarily one-sided game.
You might also want to pick up the associated record sheets for the Tech Readouts of your choice, as it'll save you a lot of time filling in little armor bubbles on a record sheet every time you want to play a game.
After poking around at the Tech Readouts, you may want to delve deeper into the rules. The Core rules are explained here, but the first one you should pick up is Total War.
As for the Great Turtle, that wiki page has a bibliography at the bottom, which tells you that the source of the mech is Technical Readout: 3055 upgrade, so that's the book you want for the Great Turtle's game stats.