It is hard to explain in words but it is something that you naturally learn over time. Positioning and role in a teamfight is key, you need to know what will be the best for your team. Either being the ADC and keeping distance from the enemies, being a tank who peels for the ADC, an Assassin who comes in from the side to kill a key damage dealer, a tank to disrupt the backline etc.
There are many jobs you have to do and many ideas to learn like this and more such as target priority. A lot of teams go for the easy kill by focussing the support but that isn't going to help you, you want to kill the damage dealers first such as the ADC, the others may seem like a problem but once their threats are gone they are just a giant sponge with no real damage.
Also, in your situation, you've got to work to snowball when you are ahead. Don't just be content with having a few kills over the champion you're laning against. Capitalise on your advantage; roam the map searching for kills or towers, take objectives such as dragons or bait the enemy team into fighting you at dragon when you know your team has the advantage.