You're on the right track just by having a main champ, and specifically one who isn't a contested pick most of the time. What you'll want to do is focus on improving your own play, both by continuing to master your mechanics and by learning to pay more attention to the positions of other lanes.
Playing top lane, you have the unique advantage of usually taking teleport as part of the meta. This means that you can tell bot lane to get a deep ward in and let the enemy push, then teleport down to get a 3v2 fight against the enemy bot lane (or 4v2/5v2 if you can get the jungler/mid to coordinate a gank). This relies on you winning your lane, though, so you have to focus on your own skill and mechanics before anything else.
Don't always teleport to lane. If your creep wave is near the enemy tower, or if it's in the middle and you're doing reasonably well so far, walk to lane and tell bot that you're holding TP for ganks. Telling them this and telling them to place that deep ward in the enemy bush will set you up for a successful TP gank. The enemy top will probably TP to lane for no value or to catch farm under tower, so you then have the teleport advantage.
This also works if you manage to get a solo kill or even a gank assisted kill onto the enemy laner. They will probably teleport back to lane, and you should just walk back, knowing that when you go to gank bot, they can't follow.
In the mid game, and into the late game, you have to decide if your team is doing well enough for you to split push. If your team is ahead or even with the enemy, and you're strong enough to duel your lane opponent and survive a 1v2, you should split push. If your team is doing poorly or if you're too weak to duel your lane opponent, you'll want to start grouping around 12-15 minutes when outer towers have fallen in most if not all lanes. When you're split pushing, you want to SAVE TELEPORT so you can join your team if they get engaged in a 4v5 that you think you can turn into a win. Sometimes, you'll just take a tower while your team dies, and then teleport to a friendly tower to wave clear and prevent the enemy push after their 4v5 win. Either way, you're getting value for your team.
As Gangplank, you can help other lanes even without teleport, just by communicating that you're going to ult so they know to start fighting, and then dropping that Cannon Barrage right on top of the enemy.
One other thing that a lot of lower rank players don't know to do - ALWAYS push the minion wave to tower after a kill to deny farm, UNLESS you are low and don't know where the enemy jungler is (and don't have a ward. That's another thing - always ward. Buy a pink for the bush you can defend, and keep a trinket ward in the other one.).
You're going to need a second role, simply because of how matchmaking works now. Since you play Top already, I would recommend picking up Mid. Pick a champ who seems like they'll be fun or who you've enjoyed playing in the past, and put in 10-20 normal games as that champ in the mid lane so that you're good with their mechanics. You'll want to roam whenever you're winning lane, which is basically the same as saving TP to gank except you'll actually have to walk unless you take teleport on your mid champ (which is valid, though I prefer to take ignite to secure kills).
Above all, play to improve yourself, not just to win. Learn to identify when you've made a mistake, and try to not do that thing again. You'd be surprised how many game wins you'll get simply because you didn't go for that risky tower dive play at level 5. Also, keep a positive attitude! Even if bot lane is feeding, if you're doing well you can carry them in team fights. Never yell at your teammates for their mistakes - they know they messed up, and they will only play worse if you nag them about it. If things look bleak for the other lanes, try to keep morale up, saying "we'll come back" or "play safe, the enemy will throw and then we'll be back to even" or some such. It really is League of Throws in the lower rankings, and the team who throws less and capitalizes on enemy mistakes more will win.
You're already on a good path since you are looking for help to improve and advance. In addition to taking advice from the forums, I would recommend some outside resources like Youtube "how to improve at League" videos, the Summoner's School sub-reddit, and occassionally watching higher ranked players stream so that you can see what they do at different phases of the game and take those techniques for yourself.
Good luck, and have fun!
and i counter pick when i can or i take a tank when i must.
, practice with him mid and top too and be good enough to carry my silver games.
