I often see newer players confused about WHAT they need to do and WHEN (and consequently WHERE they need to be.) Like others have said the general laning setup you will see is 1 top 1 mid and 2 bottom with 1 jungler. In champion select try and make sure you have a champion who can jungle effectively and has smite. Make sure you have two champions that can hold solo lanes (one capable of holding in a 1vs2.) Sometimes this doesn't happen but you ought to do your best to pick a good combination of heroes.
During the game.. some people will make mistakes and die. Let's for a moment assume all your lanes are holding ok, and not feeding needlessly. In this case everyone on your team needs to be getting as much GOLD and EXPERIENCE as possible for the first ~20 minutes of the game. Your main objectives are: Protect the tower in your lane, farm minions, don't die to ganks or bad engagements (don't make poor decisions that result in your death), control dragon (your jungler should ward this, and your team should take dragon if you manage to kill a few enemies
in bottom or middle lane.)
Things you shouldn't do in the first 20 minutes of the game:
Leave your lane to go gank another lane if it is RANDOMLY. ALWAYS favor staying in your lane and getting the GOLD and EXP that is there for you. Allow your jungler to control the FLOW of the game in terms of WHEN your team goes for kills or pushes towers.
However, it is extremely important to stress the word RANDOMLY. When I say randomly, I mean when you have no opportunity. And opportunity can arise from almost ANYTHING. It can even arise from your own dominance in lane. If you are dominating your lane, but your opponent is playing incredibly passively, and just farming while sitting ontop of his turret, you now a lot of free time to go do whatever you want, because it will take some time before you can last hit again without overextending and being a victim of a jungle gank. One of the things you can do is leave to go gank, especially if you are mid, and happen to be an excellent ganker. If any other player is not near their tower, whether it be a laner, or a jungler that you have spotted and you can reach them before they can reach a tower without you endangering yourself, you have found yourself and opportunity, and can possibly capitalize on this opportunity.
Pay attention to the enemies in lane. If, for example, the enemy champion who is soloing top lane leaves to gank mid, and everyone on your team STAYS in their own lane farming and nobody dies to a gank while the enemy top is roaming then YOUR TEAM HAS GAINED AN ADVANTAGE in GOLD and EXP. Your top solo will have more experience and gold than their top solo who made the poor choice to gank mid at the wrong time and it didn't pay off for him.
TL;DR Don't leave your lane to gank at random times. Pay attention to the map and focus on GOLD, EXP, and NOT DYING. If you can do this better than the other team you will generally gain an advantage in the first 20 minutes of the game.
Note: in high ELO games people always stay in lane and farm leaving only if their lane is pushed to the other turret, or they have teleport, or a teammate is covering the lane, or their opponent laner is MIA. Timing is everything. Make sure you don't let your opponents get a gold or experience advantage on you for no reason. Often times staying in lane is the right choice.