は is "ha" like in spring - はる. When you write in Japanese, the topic particle is は but you read it "wa" rather than "ha" (this is done so as to confuse gaijins, don't believe anybody who tells you otherwise). In konnichiwa, wa is just that particle so by-the-book you write は, but it's acceptable to write わ when texting somebody or whatever, it's a colloquial way of writing it. It's an exception to the rule, though.

As for IME, you just have to input ha, "ore wa otoko da" must be written "ore ha otoko da" to appear correctly. If you type "wa", you get a laughable abomination.