Before attacking anything as difficult as Thucydides, I would suggest getting a firm basis in Herodotus and/or Xenophon. The speeches of Thucydides are back-breakingly difficult and though the standard narratives of military movement are pretty straight forward, things like the opening bit on early Greek history are very complex in their syntax. Herodotus is much simpler and about 200 pages of him would give a good deal of fluency. Xenophon, of course, is the classic; but he is a bit dull. I have hand-written PDFs of running vocabulary to most of Herodotus. My bits for books 8 and 9 are the best to begin with (I think) and also one can follow the double-green Cambridge commentaries for advanced information. I honestly think one should work up to Thucydides in order to minimize the pain. But if one is REALLY bent on that, I myself would be glad to read the first few chapters with someone on Google Hangouts — just to help one orient.