Need some General Help for RPG Maker 2000 ('cuz I'm totally clueless)

Hi there. I'm completely new at the video game creation stuff and I'm working on an RPG starring mine and my boyfriend's characters.
I already tried RPG Maker 2003 but had too many problems with it, so I switched to the earlier version.
So instead of flooding the forum with a bunch of "I need help on this" topics, I thought I'd just create one, because since I'm new at all this I'm gonna have tons of questions.
Right now, I'm having a difficult time with sprites. I edited the cat sprite to make Mau's sprite, but I'm running into problems with it...either it shows up black when I go to select the character sprite I need...or else there's a green halo. I don't understand it, all the sprites have a green back ground and this doesn't show up.
I'm also curious about things like events. Like for example, is there a way an event can trigger a character to appear that wasn't there before, so they could join the party without having to fight them?
And years ago I attempted to use the maker before, but was only able to use four party members, how can I make it able to switch party members if I wanted to like in some of the other games I've seen made with this?
And also, I want the events to be able to change with a story, so I don't know how to make NPC characters address when another character is there and when they aren't.
Like I said, I'll have tons of questions as I work on this and I really appreciate everyone's help.
At least I've got teleporting from room to room down and a map...I might want to make more places on my map later on though. xx;

What software did you use to edit the sprite? Did you export the sprite before you edited it? That’s extremely important, If you didn’t export it then edit the exported image, you risk ruining your RTP files (probably).

After you finish editing the sprite, make sure it has the correct dimensions and that you save it in 256 colours. I use GraphicsGale for spriting, it lets you create a bare image with the dimensions and colours you want and then save as a BMP or PNG which you can then use in RM2K.

"And also, I want the events to be able to change with a story, so I don't know how to make NPC characters address when another character is there and when they aren't."

I believe I have a solution for this.

Try going into the Events on the NPC, Conditional branch. I'll use the name Matt as an example for a party member.

<>Branch if Matt in the Party
   <>Message:(NPC Message here)
   <>Message:(Example) Matt! How wonderful to see you!
   <>
: Else Handler
   <>(NPC Message here should Matt not be in the party.)
   <>
: End
<>

Hope that works out for ya.
You can work from there turning switches on and off, or creating other things that should happen when the NPC is spoken to.
Let me know if you need any help on that stuff.