I don't have any guides but a good starting point would be the following:
1) Appoint a shot caller - Someone who makes the decisions and the rest follow. Having 5 people calling out causes confusion and chaos, it's a big part of team wipes. Granted if the call that's made is wrong then by all means suggest otherwise but in general, go with the call.
2) Don't run in outnumbered even if you will lose the objective, you will likely die and still lose it but then have to deal with a lengthy cool down, especially late game.
3) Don't team fight if they have level 10 ults and you don't, 80% of the time you will be wiped as they are too powerful, pull away and soak until you hit 10.
4) Target the squishes. Too many people will focus warriors as they're usually in your face, all this does is provide the enemy teams damage dealers with free reign to take you out. Decide before you engage who your target is then all focus on that hero. My advice go for the highest damage dealers, if you remove their damage they can't do much. If no assasins near by then go for their healer, always leave the warriors until last unless you have no choice.
5) Your front liner (warrior) always initiates the fight.
6) Keep moving, don't face plant while standing still, you are an easy target for skill shots, this also applies while laning.
7) Be aware of your surroundings, if you have lots of aoe damage then try funnel them into a small space, they may do the same, if they have lots of aoe, don't get caught in tight spaces.
8) Keep an eye out for any shimmering, this will be a cloaked hero either Nova or Zeratul, once you see it you get used to spotting it.
9) Dont get carried away, you may kill a few then a rush of blood to the head sees you chasing one or two enemies as they run away, on many occasions you end up too deep in their base and can easily lose a few heroes. If you wipe a few and 2 start to flee, if you can't grab them within a couple of seconds, pull back and regroup. Getting caught out of position happens a lot.
10) Look at your team comps, see what combos you can play off each other.
11) Have fun, more you play the more you will understand objectives.