This is a collection of 9 tips for every Dota starter. Impersonating those tips should make your game play much better and allow you to start your owning streak. Feel free to add your suggestions below.
1: Do not save for late game items at the start.
It's not of any use to safe for late game items right from the start. Make sure to buy early game gear because that way you are able to farm more and eventually even get hero kills. For every hero start with a Wraith Band/Bracer/Null Talisman. However, do not buy the full recipe right from the start of the game, solely buy the individual attribute boosters! The recipe is not worth it at the start! You may also go for a bottle. It's totally worth it, calculate you get 2 creep kills more a minute due to your starter items, that's 20 after 10 minutes, or 900 gold. For almost all melee heroes, buy a quelling blade!
2: Do not Auto Attack creeps.
You read right, that's the worst thing you can do. This may sound annoying to you, but run around the creeps and solely attack them when they are low HP. That way you are getting way more gold right from the beginning, this will pay out late game. Also deny creeps, if properly done you can level up to 7 whereas your opponent is still on level 4. Make sure that health bars are enabled in your Warcraft 3 settings. Same applies for towers, try to get the last hit on the tower, it's worth it.
3: Do not waste all your mana before level 3.
Your damage/mana point ratio for level 3 spells is way better than for level 1 spells. Wait till level 3 until you attempt to nuke your opponents. Once you are on level 3 you can attempt to nuke them.
4: Do not always run - fight!
There are many situations where it's better for you and your team in case you stay and start fighting the person trying to gank you. It may even be the case that you are able to kill him. In around 80% of the situations it's useful to stay and try to fight back. Exception is of course in case you can blink away.
5: Multi-tasking - watch the other lanes.
In most public games you do not have a good team announcing all the misses. Make sure that you check yourself and adapt dangerous situations. It's called mind gaming, use your brain and start owning.
6: Make use of the fog of war.
The fog of war is the mightiest weapon your dota games. People are following you and lose sight, that means that you are able to perform unexpected moves, like for example casting your ultimate, going an opposite direction, there are a lot of opportunities. When being followed you can walk through trees, walk back around, buy a TP for example and stay a live despite impossible situations. Try it out.
7: Read Dota guides.
You will need to understand how different items work together, how heroes work, what they are able to cast and more. By reading about different heroes you are able to learn about the game and adapt to every situation and game mode. Here you can find a good DotA Guide by !Red!.
8: Buy Disablers.
In some situations it may turn out better for your team if not all are egoistic and go for their individual item build. It may be better to go for disablers in order to disable strong end game heroes.
9: Don't leave, stay and try to turn the game.
Many of the people I am playing against rage quit after seeing that I start owning them. Stay cool, think and buy items that are able to easily top the other hero. Eventually buy disablers, with good teamplay you can turn every game.
I hope this should give you a pretty good idea of what to do in your Dota games.