I took at a peek at your lolking profile; my champion of choice is 
, so if you like his play style I can do my best to provide you tips! :) I'm still working on my execution, but he's the champion I am most comfortable with by far. Beyond that, what is the biggest trouble you're having?
I'm learning a LOT by observation. I'm still very low in the scheme of things (only just hit level 28), but by watching others, taking notes, and doing LOTS of practice, I'm gradually improving.
Sit down for a couple of hours and watch a ranked game or two in the client, or watch a skilled player on youtube! Find a champion you play or you're interested in and watch them. Take notes; how are they building, why are they building that way? Where are wards being placed, how is their map awareness? What mistakes are being made that give them or the enemy an advantage? How do they come back from it?
Learning by watching others-- and understanding why they do what they do-- is amazingly insightful and helpful. Put it to practice, and take it one step at a time. :)
EDIT: You may want to peek at your masteries, too... they're super split up, and it might be best to focus one tree first; like offense, for Heimerdinger. The further down on the tree you get, the better it is. Try something like this maybe (obviously I'm still building, but the next two points would be in defense for Veteran's Scars and Juggernaut). You might want to peek at your runes, too! It might not seem like much, but it can make a huge difference. Play around a bit, see what others are using, and try them out for yourself!
And again; not claiming to be amazing or anything by ANY means, just trying to share what I've found that's helped me a ton! I hope some other players will be able to jump in and give you some better advice, too. :)
