It literally is "this evening" with the topic particle (which can be seen from the kanji which is 今晩は). This also explains why it is pronounced as "wa" and not as "ha". I think it comes from a full greeting (something along the line of "as for this evening, how are you"). This greeting started with こんばんは and eventually it became a greeting on its own. It is the same thing with こんにちは which literally means " this day" with the topic particle. (The kanji for こんにちは is 今日は).
In fact, it is not easy to identify a は (or other particles) is a particle. Some experience is needed and often we need to identify the nouns/verbs/adjectives before we can identify the particles. If we can memorize more words, then the particles will generally come out. Writing the words in kanji, or separating phrases with spaces will help.
This is a phrase that was broken up to this part and the rest was left out. The whole phrase is not used anymore... But you could sig for the info online. The whole point is that the は is pronounce wa because is part of the phrase that has a particle on it and the particle is always pronounce wa.
Basically, 今日は。konnichi wa 今晩は。konnban wa. (even it is not really asking you) came from the questions 'How are you 今日(konnichi)?' (this day = today) ? How are you 今晩(konban) (this evening) ? as david.tole3 said. Therefore those expressions are used when you see someone, not leaving them because you already know how they are.
you've probably figured it out by now, but if you'd still like to now, こんぱんは (konbanwa) is read like con-bun-wa (the n's are a little less pronounced, they should not cause your tongue to touch the teeth) and お休みなさい (oyasuminasai) is read kinda like oja-sue-me-nuh-us-ay (the j in oja is like a y, i really can't help you more through text lol)
I think the best thing you do is either looking up the IPA for them, or listening a native speaker pronouncing these words, if you can speak spanish or some other romance language, it would be easier trying to mimic their sounds for the vowels, like how "a" has always a very open unrounded sound (close to how we pronounce the letter "i").
hope i helped ¯_(ツ)_/¯