Help me be less terrible while PVPing (Sentinel)

I'm pvping to clear a certain quest. I play a Watchman Sentinel. Rerolling is not an option. :)

I expect to be pretty bad as I've only done pve stuff with this character; my SWTOR PVP experience was as a Sniper back when the level cap was 50.

I'd like to be as less-bad as possible while doing these 20 matches. I've heard the Bolsterizer is broken ATM, but I've gathered a set of 190 rating gear which should give me max Expertise?

Does anyone have any tips on what I can do to be less of a liability for my team, particularly in those 4v4 arenas? It sucks that I 'have' to PVP to get this,quest done but it seems rude to not at least try my best and be a contributing part of the team.

Edit: just a huge thank you for all the advice, tips and encouragement. I actually gave it a go this afternoon and while I had no real idea what I was doing, I stuck to calling inc and protecting a healer. Team fished out two wins, I'm sure in spite of me, and I was able to get PVP weapons (which should bring me to 2018 expertise, as it seems crystals aren't bolstering).

I had something like 400 expertise according to the Bolsterizer on Fleet but 1936 inside the warzones, so something is wrong there. :p 190 gear worked fine, with the exception of crystals, as noted.

Thanks again!