AD Carry and Marksman are terms for the same role.
More important than the distinct champions to buy is that you get a few reasonable rune pages set up. For starters, the standard AD page is AD reds + quints, Armor yellows, MR glyphs (all runes flat). Standard AP page is exchanging red for Mpen or Hybrid pen and quints for AP.
From there on out, I really recommed buying the 7-page bundle. Plenty of space to work with.
For example, a more jungler and AD carry specific rune page would contain 2-3 AS quints (and one quint of either Armor or AD), AD reds, either full flat Armor (jungler) or 4 Armor + 5 HP yellows and flat (strong magic damage poking support) or scaling MR (tanky enemy support) quints.
A variation of the AD rune page could contain scaling MR over flat, if your direct lane opponent does mostly physical damage.
For champions, you mostly need to find what you like for each position. Currently perceived as strong champions include:
Top:
Jungle:
Mid:
AD Carry:
Support:
Still, you personal skill and comfort level with a champion mostly trumps inherent differences in champion damage.
Btw, Tryndamere isn't a good jungler - he works way better as a top laner.
Good websites for guides are LoLKing and solomid.net. Mobafire, not so much.