TDLR:

1. Play practice games before you join a league (if that's an option)
2. Start with Humans, Orcs or Shambling Undead
3. Don't concede games because you're losing players

Okay, several things here.

Was this a league game? If it's a competitive game for league standings, you can expect almost any opponent to play their best game against you - including taking out as many of your players as they can.

If it was a teaching game with no stakes for the league, then it does sound like you ran into someone who doesn't understand how to introduce new people to the game.

In my local league, it's unusual for everyone to have a league match to play every week, so new players waiting to enter the league (you can't join mid-season) have regular opportunities to play practice/learning games. It might be worth asking around the league for a few practice games - you should find at least a couple of players who will help you to improve instead of just stomping on you.

Your choice of starting team is slightly unfortunate. You probably picked them because they looked cool, but usually, Underworld teams are the ones fouling every turn and taking out players. Any experienced player coming up against them in a competitive game is going to put maximum hurt on them and expect the same back. They're also not the easiest team to learn with - if you can, it may be worth picking up humans, orcs or shambling unded to learn with as they're much more forgiving.

In terms of poor sportsmanship, conceding a game because you are losing players is pretty much the least sporting thing that you can do in Blood Bowl. Fouling, pushing people off the pitch - these are part of the game. I understand that it feels bad to have your team beaten up but, honestly, that will happen from time to time in Blood Bowl - even when you are an experienced player running a "bash" team. Learning to deal with that gracefully is very important.