What's the diference between konichiwa and ohaiyoo?
The first one is more like "Good afternoon" while the second one is "Good morning." I'll also correct you here so you don't develop a bad habit. "Ohayo" おはよう. There's no "i" in it, even though it seems like it sometimes.
As for the numbers thing, it's pretty simple if you just want to say your numbers. However, be aware that if you want to say there's a certain number of things or people, the rule changes a bit. Don't worry about it now since you wont get to it for a while.