Unlike some people around the boards you're already a few steps ahead. You know you're bad. Believe me when I say this knowledge will help you immensely. I've been attempting to help someone for a few months now who refuses to accept that he plays at a bronze level...
First thing to understand is that bronze and silver, even though they are the lowest ranks, are the most populated. Over 50% of players, as shown here, are in those two divisions. You're not as far away from being a player of average skill than you might believe.
CSing is the easiest thing to practice, since it can be done in a custom game. Take a particular champion, use an empty rune and mastery page, and head to lane with no items. Just practice CSing until you hit 10 minutes in game, then stop and restart. You should aim for 100 CS at 10 minutes. Personally I'm only around 80 at the moment as a high silver/low gold player. I've heard it said that around 70 is decent for people outside of high ELO, so once you get close to 70 you can slow down the CS practice. Just remember to go back and practice from time to time, it really helps.
The next thing you should do is theorycraft. This one is fully my opinion, but it really helped me improve back in the day. Spend a few hours making custom item sets for your champions and really take the time to think about why each item is useful on your champion, and why a different choice may be better. Each item has something called gold efficiency (how efficient it's stats are compared to both it's price and the cheapest item that gives each particular stat) and slot efficiency (how effective the stats are for each individual slot). You can only have six items, so do you want to focus on gold efficient items that give you early game power, or slot efficient items that may be more expensive, but will help you more come late game?
Next do the same thing for your runes and masteries. Knowing why something works is far more important to you than simply knowing that it works. Guides can give you a good starting point, but a lot of people end there too. Once you know why a choice is good or bad you an start making decisions specific to your own play style. You're not playing in the LCS, so a lot of things that are technically suboptimal choices could be just the thing you need. Personally I use a lot of scaling runes which are almost universally considered worse than flat runes, yet I don't play champions with strong early games. I chose scaling to boost my mid game power spike, rather than an early game one. Optimizing your runes and masteries (and items) to fit your particular style of play can be a huge help.
Lastly don't let yourself play while tilted. If something is keeping you from playing your best you can develop bad habits that are very difficult to get rid of. After a frustrating game take a few minutes to move around and get your blood pumping. Get a snack or a drink or anything else you need, or just take a moment to get some fresh air. Starting every game with a clear head will, obviously, be a benefit.
As for stuff like trading I'm afraid I can't help you. Trading requires you to fully understand the amount of damage your champion can deal and take relative to the damage your opponent can deal and take. The meta is always shifting, so this is something you'll constantly have to be relearning. Just learn your champions as best you can and spend the very early game testing your strength in as safe a manner as possible. Just remember some champions have insanely strong level 2 power spikes, so be ready and good luck.
