I highly dont recomend u duelists for a begginner, because it's a role that you won't know, regardless of whether you came from csgo or another game and it has to have an impact, something that probably in the beginning, you will be afraid of wanting to go out killing enemies... So, best options are:
Sage - Support agent, u just need to know that sage wall is for block enemies and u can close some map locations, quite easily... also, throw sage slow orbs to prevent enemies to rush on you... other thing, is just try to shot back ;)
Kayo - dsnt matter if u just throw kayo flashes like a maniac, for a begginer, ppl dont know how to turn on flashes, so u just need to use kayo skills, u don't even need aim at the beginning, if you use a minimum of his skills. There are several tutorials which you can learn easily to evolve in the game.
Brimstone - Smoke agents are tricky... but brim is the easiest, by far. U just need to learn how to smoke with his watch... and, arrange the minimap correctly in the game, look for explanatory videos of settings
For a sentinel role, u can pick Killjoy, just put kj mollys on spike, and engage fights with your turret, never forget to put alambot on your back. U can do fine, i guess...
Buttt, if u want a duelist anyway, please dont go for reyna, its a one trick agent, (Flash + Shoot), and u will not be able to take advantage of that. In any hypothesis go for reyna... The best option is still Phoenix, u just need to learn how to use his flashes, bcause u can get blinded quite easily, other than that, just go with your teammates and try not to flash them ;).
BANNED OPTIONS: astra, jett, chamber, sova, skye, deadlock (dont need to ask why, just dont go for these, cuz is too difficult... even immortals cant play well with these agents)...:
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