Can someone help

So it will be our first time ever at Disneyland. I’m trying to buy tickets but we’re only going for a day. I wanna get tickets but I’m not sure why it says two different parks? I don’t even know which one to pick 😭😭 I’m confused It will be during the weekday here in LA . Any recommendations or tips will help a mom out of 2 toddlers and one baby 😭😭

Can I also get them when I arrive?

Do I have to make to make reservation before I go?

Is it expensive? Like the tickets 😭

Ahhhhhhh