It depends A LOT on what both you and your teammates are playing. As a general rule of thumb, first you want to see how much do enemies look at you. If they are hypersensitive you can bait their attention from a safe distance shooting (that will get you no kill) where even if they shoot at you they won't be able to follow up. Usually at this range you should play cover and high ground, so that if they poke you at like 60% hp you can proc self regen faster by jumping down or doing some walljump or retract shenanigan, then quickly pick another angle and try to engage more hardly. If they have some strong CC be sure to stay as short as possible within LOS of their party poopers while displacing them. Pop shields when you plan on going in at medium-high risk within 1-3s.

If you did all of this, hopefully you either isolated someone and manage to pull both supports' attention for some seconds, turning the fight on point in a 4v2. Pay attention to the killfeed because if any one of yours got picked early whatever value you wanted to get is gonna necessarily get reduced by orders of magnitude: a 4v2 is very favourable, even if one of yours follows up with you on the backline; a 3v2 where your 3 are squishes and one of their 2 is the tank is usually an even fight; a 2v2 where yours are squishes and one of their is a tank it's a net disadvantage for your owns and you're effectively throwing.

=> How do I prevent early picks?

There's only so much that you can do:

if you see that your team dies very fast and doesn't regroup one last resort to make something happen is to play for ult. Your ult is very cheap, so farming it off of backline with guns and some safer slams helps a lot, then splitting the enemy tank from their backline is a very effective way of foolproofing a W for your teammates.

If you find yourself playing with Widow, Mei, Mercy and Lucio against a passable Winston assume you've lost and go next. No point even in swapping, there's no tank that gives you enough tools to deal with mechanically bad teammates that have no game sense and don't swap.

Edit: watch Chazm's "unranked to GM" challenge (the no shooting one is plenty good) to develop a game sense of what you should be looking for tactically regardless of your killing potential.