1. They just do, and you are going to see that more. Especially with counting. It has to do with the way it sounds on the ears, and Japanese rely on sounds more than pronunciation. So it is going to have the hiragana ha, but ending with wa sound instead.
2. Yes, it means 'my dog'. With that knowledge, you will be able to bring up subjects of what you own. There is a bit more of a complex form of it where you declare you 'own' something more than regarding it as 'mine'.
3. "This is my car, and..." This is pretty advanced. Why you ask? It has the て/で form. This will be a very complex sentence structure later that will elongate your sentences. This also act as attachments to verbs within the sentence structure. これ belongs to a list of object pointers near the listener, and can be pretty much controversial how they are used. There are also the version to point out something more than one object, and last but not least...When I mentioned about the SOV (Subject Object Verb), this sorta demonstrates how the て/で forms come into play. As you might as guess: です is going to do that later, it will become 'and' to some things.
4. Yes, it is. You will have ます, ました, ません, ませんでした. Those are polite ending actions to verbs that do current and past tense. But です alone is just a cherry on top, as I said. And it must come ONLY after the whole sentence is completed, not during it. Otherwise it comes off as funny.
Okay, since I explained that, greetings since I did not cover this.
The most common greeting in Japanese is こんにちは is the most standard usage, and can be used at any time. Though it is also known as a 'noon' greeting. It does not need 'desu' at the end, and is easy to catch on. Another one is もしもし which gets used on the phone, or to get someone's attention. It is not impolite.
Now we get into early morning, you want to greet someone for the very first time. You say: おひようございます. The more causal is just おひよう.
Evening is こんばんは.
Good night is おやすみなさい.
Now for good bye can be very odd since people think さよなら means just good bye, no. It means good bye forever till we meet again, or some months after. It really literally means that.
http://blogs.transparent.com/japanese/how-to-say-goodbye-in-japanese/
Here is a link explaining about the context of saying good bye since there is a LOT for me to usher here.
To top it off, here is a site explaining how your first meeting with someone goes. It kinda saves me the time to explain it out because off, lengthy...
http://www.freejapaneselessons.com/lesson10.cfm
I think I covered most what you will be learning for now. It is a lot, but it will help in a long run.
(Edit: The が particle is used when the listener is aware of something, and is used with things such as これ. They are never, ever paired up with the particle は, and are also paired with actions and directions. Keep in mind of that when using it.)
(Edit: だ is not just a male term. However there IS terms for both male and female, but it is just a 'tact' on thing. It is just to sound super masculine, or super girly, otherwise not all Japanese use it. Same goes with the pronouns.)
