There is no longer such a thing as a 'touch attack'.
From the linked question: Touch only indicates the range, the target has to be within your natural reach (5' for a medium or small character).
If the spell says it requires an attack, that is a spell attack, not a 'touch' attack.
Spell attacks are d20 + proficiency bonus + spell attack modifier, resolved against AC in the standard way. Within the current rules, a creature has a single AC value which doesn't change depending on the type of attack.
You can look up the rules on spellcasting in general and spell attacks in particular in the Basic Rules available for free use here.