After Harry and Ron discovered Hermione's note (torn from a book) and they figured out where the Chamber might be and that Slytherin's monster is a basilisk, they went to the staffroom. My question is why didn't they ask for help from a professor like McGonagall or Flitwick and went to Lockhart, instead.
I don't think that "because he is the DADA teacher" is a good answer, even if Harry says that when Lockhart refuses to help, because they knew that Lockhart was dumb. Moreover, when they arrived in the staffroom, professors Sprout, McGonagall, Snape and Flitwick were mocking Lockhart.
I believe that prof. McGonagall for example would've been incredibly useful in the Chamber of Secrets.
Regarding the fact that they believed that Lockhart will try to enter the Room:
Snape: Just the man.[...] Your moment has come at last.
McGonagall: That's got him out from under our feet.
Lockhart: "I - well, I - " sputtered Lockhart. "I - I really never - you may have misunderstood - "
Lockhart gazed desperately around him, but nobody came to the rescue. He didn't look remotely handsome anymore. His lip was trembling, and in the absence of his usually toothy grin, he looked weak-chinned and feeble.
I think that they had enough clues to figure out that Lockhart couldn't help them.