Auto Roll Max Damage On First Crit Dice?

I'm fairly new to roll 20 and can see that on 5e character sheets there is an option to somehow alter damage dealt when players get crits with their attacks. Is there a way to set up the character sheets using that tool (or another tool) so that when players roll crits the first dice automatically does maximum damage?

You'll have to modify each attack separately, and it's way easier to modify the 2nd die as that's the one that only shows up when a crit is scored.

Go to edit an attack entry on the sheet, go to the Crit field, which is normally blank, and then type in the max value of the attack's damage di(c)e. So, for instance if it's a rapier, you would put an 8 in the Crit field. Then, when a crit is scored, it would display the damage as (1d8+modifiers) + 8 Crit damage.

If that max value doesn't apply on savage crits from half orcs or barbarians, then put the weapon damage die+ the max value in that field, so for a rapier again you would type 1d8+8

Sadly this cannot be applied to NPC sheets as they don't have crit fields (at least, not ones that can be edited).

You can use a slightly hacky solution for the NPC sheets, if you wanted to edit those as well. Using an attack that does 2d6 + 5 as an example, you'd put this in the description field of the NPC's attack (if there's already a description in there, this goes on the end):

}} {{crit1=[[6 + 1d6 + 5]]