What is my job as scorer, and will I use paper or an app?

Either is allowed. Under USAV Rule 25.2, the scorer fills in the score sheet or an electronic scoring device according to the rules, working with the 2nd referee. USA Volleyball publishes paper score sheets (non-deciding one-set and two-set sheets and a deciding-set sheet) and supports the VolleyStation Score app for electronic scoring. The tournament decides which is used.

Your duties (USAV 25):

• Before the match: record match and team data, including libero names and numbers, and each set's starting lineups. Captains and coaches don't sign before the match.
• During the match: record points, control the serving order and report a wrong server right after the serve, acknowledge and announce substitution requests with the buzzer, record substitutions and timeouts, flag out-of-order requests, announce set ends (and the 8th point in a deciding set if courts change), and record sanctions and anything else the 2nd referee asks.
• At the end: record the result, sign the sheet, then get the 1st referee's signature.

You sit at the scorer's table facing the 1st referee. The assistant scorer beside you tracks liberos. At USAV tournaments these roles are often filled by an assigned work team; USA Volleyball recommends that certified scorers act as scorer.