First piece of advice, don't limit yourself to only 2 champions. Try to make a list of 5 champions you are either comfortable with or that you want to learn/master and only focus on those 5. Poppy and Mao'kai are strong, but try tossing in 3 others to still keep that variety while keeping yourself focused. Being a 1-trick is not a good thing. Also, if you are being placed in another role for ranked try to make 2 of that specific role to your 5 that other role. The main reason for this is so that it is difficult for you to get banned out and it makes it so that you HAVE champions of that off-role. If you like Tanks/heavy bruisers, consider adding Sion/Olaf/Fiora/Jax to your top lane roster, if you play support, take Alistar/Braum/Leona. Variety is not a hindrance, but specializing in a select champion pool will help you in the long run.
Second piece of advice, at the beginning of any game MUTE all members of the enemy team (both Chat and Emotes). This will make it harder for them to Tilt you and also keeps chat clear for your teams pings and call-outs to be seen.
Third piece of advice, watch a few videos of CURRENT gameplay for the champions you play or want to play to see how others: Position themselves in trades/ganks/teamfights, vision placement and map awareness, CD's of your opponent in lane (ult/engage abilities + summs), Roaming/counter play on OBJ/tower/teamfights, Core Items vs situational items, and how they CS vs oppressive/passive lane. I know this is a lot to understand and check all at once, but a lot of it will become instinctual as you play more.
Fourth piece of advice, always assume the person/team you are against is better than you are. This sounds dumb, but you will play more cautiously if you do and you will be less likely to died from overestimating yourself. I can't tell you how many times I've watch a teammate lose lane simply because they though the lane was "easy". Even if you are in a favorable match up, if your opponent understand their mechanics and potentials of their champion better than you do they can still win.
Fifth and final piece of advice, keep a cool and calm mentality while playing. Don't over hype yourself after a win, don't get depressed after a loss. try to only play games in sets of 3 or 5 and never more than that. These "sets" are simple to follow, if you win 2/3 games in a set of 3 you had a good gaming session and you can keep going, but, if you go 1/3 stop playing for an hour or so and allow yourself to "reset" your mentality on the game and try again. Same holds true for the sets of 5, 3/5 or bettter keep rolling the games if you want, 2/5 or less stop playing for a little while and come back. This holds true for the game as a whole, regardless of gametype you played in.
Keep all of this in mind as best you can and I'm sure you will at minimum reach high Silver / low Gold.