It depends on your champ and your opponent. If you have good wave clear and can shove the wave to the turret then you should immediately look to roam top or bottom and pick up a kill or force a summoners. For example, you shove your lane, look at your minimap. If the opponents bottom lane is at the river at the least, run down there and try to do some damage. The time is not wasted if you force them to flash or use heal or something. This does two things: 1) It allows you to roam bottom again and now no one has flash or other mobility and you can secure a kill easier. 2) It allows the jungler to do the same thing! If you happen to get a kill on your initial roam, the better.
Shoving minions to the tower also forces them to potentially lose CS. Some champs are better than others and farming under turret. At your level of play, everyone is bad at cs and under tower it's even worse. Just remember, every 2-3 waves worth of minions is equal to a kill in gold. If you focus, early on, in getting your creep score as perfect as you can you'll be better than a ton of other players. Even if you play safe and don't make any fancy plays and die a few times, the more CS you get the better you will grow during the game. As long as you know when to start grouping and team fighting you'll always outscale your opponent if you can CS better than them. The lower your ranking, the stronger the difference is between a good CSer and a bad one.
I hope that makes sense.