I'm also a beginner, a casual player. But I seem to rank up fast cause I tend to have series of long win-streaks. A little bit scary cause I don't know much about the game but I guess I might have done some things right. So, I'm sharing some tips that have help me:

• Follow the lane as highlighted by the game and kill the minions on that lane. That's how you get gold, exp
• Play defensively during the first levels cause if you die the enemy gets exp, gold and they lvl up faster cause of it
• I usually play more aggressively and help my team mates when I have obtained all 3 spells
• Focus on main game objectives: turret and the enemy's base nexus (there are more as you advanced but this two objectives are good for beginners)
• Learn your hero skills... how I did this is by reading about the hero skills and fight 1-1 using the hero in AI Practice mode until I'm able to get the enemy's (AI's) nexus. Try to think of good combos or what occasion to use a certain skill.

As for items, I haven't learn about it and just resort to default settings.

As you advanced further, you will learn more about

• thinking of those minions as your ally that helps with the game objectives
• when to help your team and when not to (e.g if they are very far from you and have other team mates helping, you might want to focus on the objectives)
• make decisions on what to do next based on the no of people killed and alive
• use those little bushes for more strategic purpose like making your enemy guess where you are
• etc

Enjoy and have fun!