Dota 2 is famously hard to learn. These basics matter more than any build.
Last-hit creeps
You only get gold if you land the killing blow on a creep. Practise in a lobby until you can last-hit consistently. It is the single most important skill in the game.
Deny your own creeps
Attack your own creeps when they are low to stop the enemy getting gold and experience from them.
Watch the minimap
Look at the minimap every few seconds. If enemy heroes disappear from their lanes, assume they are coming for you.
Do not die for nothing
Every death gives the enemy gold and costs you farming time. When in doubt, back off to your tower.
Buy the basics
Regeneration items early, boots for every hero, and a Town Portal Scroll at all times so you can defend or join fights.
Play a few games of Turbo or unranked first. Ranked play unlocks after a number of games.