Lots of questions here, and most can be answered if you do a search on this forum and also the ESL Discussion Forum. There are posts on books to buy or borrow,discussions about different courses and their merits, and lots more. The "A" in CELTA stands for Adults, so if you want to teach children specifically then you could look at other TESOL/TEFL options.

"I'm a high school graduate...
Can I start working when I'm done or I need to gain another university degree?"

These two statements are confusing. If you have a university degree already you will not need another university degree and you don't need a degree to enrol for a CELTA course. AS for DELTA, from memeory. you will need a minimum of 2 years full-time teaching before being considered and I would recommend more years in teaching as it can be a tough course.

Having "... a passion ..." is great, but you really do need to know your stuff be be a competent teacher.

Best of luck.