Don't be caught up *always* in the backline. A mistake a lot of noobie sombra players do is they spend their time running around putting their tp on a healthpack rinse and repeat... sure you can do that if you time it well...
BUT in a team fight, you NEED to be there in order not to leave your team fighting a 5 v 6 while you're translocating all around the map.
Also, as much as possible always try to get the high ground, you'll find a surprising amount of people literally ignoring you at best and being distracted at worst when you're on top of them.
As you climb the ranks, try not to keep your TP on a healthpack always because it's gonna be very easy for a team to rush your healthpack and take you out.
I made it to 3500 playing sombra only.... well ALMOST, Sombra is good in most situations (she's not THAT situational), the times I had to hard switch was when they had a good hanzo or when we needed something else in the team comp.
Lastly but not least, in most uncoordinated public comp games, you wont get the chance to initiate with the big emp, rather you should wait for the team fight to take place and *then* emp... it's better to emp 3 targets and get the kills then have a 6 men emp and no follow-up.
The last point is ESPECIALLY true if you don't have a lucio in your team.. if you dont have a speed boost dont even think to translocate far away to emp.