To become a member of ARES you must possess at least a Technician Class amateur radio license. Following are resources to help you obtain that.

Popular study websites:

ARRL Technician Resources: http://www.arrl.org/getting-your-technician-license
HamStudy.org (free): https://hamstudy.org/
HamRadioPrep.com Tech course ($35): https://hamradioprep.com/technician-license-course/
No-Nonsense Study Guides ($15): https://www.kb6nu.com/study-guides/

Exam review websites:

https://www.arrl.org/examreview
https://hamstudy.org/
https://hamexam.org/
https://www.hamradiolicenseexam.com/ (not free)
https://hamradioprep.com/ (one practice test free)
https://www.qrz.com/hamtest/

Recommended YouTube channels:

ARRLHQ YouTube Channel: https://www.youtube.com/user/ARRLHQ
Ham Radio Answers, from David Casler KE0OG: https://www.youtube.com/c/DavidCasler
Ham Radio Crash Course, from Josh KI7NAZ: https://www.youtube.com/c/HamRadioCrashCourse
Ham Radio 2.0, from Jason KC5HWB: https://www.youtube.com/c/HamRadio20

“Old-school” materials (books, CDs, DVDs):

ARRL Ham Radio License Manual ($32.95): http://www.arrl.org/shop/Ham-Radio-License-Manual/
Pass Your Tech – The Easy Way ($17.95): https://www.amazon.com/Technician-Class-2018-2022-Amateur-EasyWayHamBooks/dp/1985125641
No-Nonsense Technician Study Guide ($17.97): https://www.amazon.com/Nonsense-Technician-Class-License-Study/dp/0983221634
Ham Radio School Tech Course ($22.95): https://www.amazon.com/HamRadioSchool-com-Technician-License-Course-Turner/dp/1477595457
Gordon West’s Technician Class ($24.95): https://www.amazon.com/2018-2022-Technician-Class-Gordon-West/dp/0945053908

(Prices listed above are paperback books from Amazon; note that Kindle versions may be available for lower cost.)