Talk to the greeter on the left in the World Tournament building (wearing green) to find out about the different varieties of tournament that happen here- single, double, triple, and rotation battles are all possible tournament formats, requiring three, four, six, and four pokemon from your team, respectively.  Driftveil's tournament is the only one open right now, though, so join in there.  The receptionist asks you to pick your team members who will participate, so choose the three that you want to have compete.  Make sure you've got a variety of options available- pulling three physical fighters or three pokemon of the same element, for instance, will result in a very easy time for any computer opponent that happens to come up with a single pokemon that's particularly good at handling whatever your team members have in common.

As it's three pokemon you're registering, you know that this will be single battles, so team moves aren't an issue, and you can eschew using any particularly weak or overleveled pokemon (and should, as your low-level team members need a chance to catch up!).  There are no restrictions on which pokemon you can have join in or what items they can be carrying, so set out to field a balanced team with good hold items and you should do alright.

With registration complete, you are ushered inside and joined into the tournament.  Before it begins, you'll get a look at the competitors' brackets- you will see that Cheren and Hugh are present on your side of the bracket, with you facing off with your old friend first of all.  Colress is also involved, on the far side of the bracket, so you can probably expect to face him last, should you reach the finals.

When the tournament starts, you'll get a view of a packed arena, where you are walking up to the battle platform past a cheering crowd.  Introduced as 'a newcomer', you then get to watch Hugh make his own approach to the platform, with the announcer declaring that Hugh is 'pushing his limits in order to find his important pokemon'.  The two of you then face off on the platform, and the bigscreen behind you shows both yourself and Hugh.

The brunet comments that Clay is apparently really big on loud and flashy things, then starts the battle.

Battle: Hugh
Opponent: Starter (Evolved once) Lv 25, (Simisage/Simipour/Simisear, whichever has type advantage against his starter) Lv 25, Tranquill Lv 25

Hugh is very fond of Detect and Roost with his Tranquill, so it will tank a surprising amount of damage if you don't hit it with Toxic poison or stat-reducing moves.  Beyond that, his other two pokemon are pretty standard for what they are, so there shouldn't be any surprises here.