I'm planning to read the manga but I don't know where? What's the best source? I prefer the closest english translation to the original.
Kodansha Comics as they are the official translator for Attack on Titan in English and license to distribute it.
It looks like you can read Chapter 1 for free but each volune does provide links to where you can get them (Amazon Kindle, Nook, iBooks and Printed (Physical) Editions)
I prefer the closest english translation to the original
without going into legally questionable grounds such as scanlations, this is your only english translation.
even then Japanese to English is not an exact thing and how close the "english translation to the original" is subjective since some people will say the use of senpai/kohai as opposed to senior/junior or retaining honorifics like -chan, -kun and -san is closer to the original