Good details above. To continue to answer your original question, you in AROTC or NROTC will receive a monthly stipend for each month of active service only. I think in the last 12 months, my DS received 9 payments. None for May, June, July. Expecting the same 9 months, not 12 months of stipends, in the 12 months ahead.
Army ROTC scholarships also provide a $420 per month living allowance for each school year. Non-scholarship Cadets in AROTC advanced courses (3rd and 4th year) also receive this allowance. As others noted, the Navy national scholarship winners receive 250, then 300, then 350, then 400 in their 1st through 4th years for the active months only in college.
As to the when the money is available - , it varies - All payments including books, tuition, fees, your monthly stipend are not activated until you show up and ultimately pass the personal fitness standard which activates your scholarship. Those who don't have until the end of the semester to make the standard. But if they don't make the grade,, as others stated, you will be on the hook to pay it all back. There are also of course discipline, academic, uniform, participation standards that you have to maintain.
Just remember too that if you commit on day 1 of your sophomore year, and then drop later, you have to pay the scholarship back.
The book stipend varies - last year some students had 1/2 of the annual amount it on day 1 of each semester, others had the stipend later in the semester - months later. Look into chegg/used book rentals on amazon etc as by doing this my DS had more given to him in the book allowance than the book rentals cost. If you cannot afford to cover the books, tuition before these payments come in, then talk to your ROO and bursar office tomorrow - confirm exactly what you are responsible for or have to worry about. Some schools will wait for the scholarship to kick in on good faith, - others expect you to pay it then be reimbursed. I was actually pulled from class my first week my freshman year because my tuition money was late - I was not allowed to go to a class until it was fixed. Some schools are patient - some are a no pay, get out policy. Just know ultimately though you're responsible.
Hope that helps and please let us know if any additional details would be helpful, Come in prepared with a resolved attitude and being physically fit and focused. Covid or not, remote or not, ROTC has standards and I've seen a lot of good young people underestimate those demands or fail to prepare and wash out. Good luck.