This guide will focus on providing advice that will help players of any skill level improve their game play using any hero, it will not be specific and focus on any one area of playing Dota nor will the tips here work for specific heroes and be useless for others.
So let's begin.
1- Always carry a Teleport Scroll, in fact always carry two of them.
It's rather baffling how players can ignore this item, for 135 gold you can teleport to any building across the map, this allows you to turn around fights that are happening far away as well as give you an amazing escape mechanism. It does take 3 seconds of channeling time to teleport away safely and sometimes you won't be able escape but regardles it's still an item that belongs on every hero's inventory.
Another thing that players should consider is having two Teleport Scrolls with them, why? because if you have only one and you use it then you won't be able to teleport again till you buy one from the side shop or the base and in that time anything can happen, so always carry around 2 Scrolls in case you'll have to teleport to two areas within a short amount of time.You also can't predict how the game will go and you may not be able to afford a TP scroll later when you really need it, so buy 2 when times are good.
2- If you're thinking of trying a new hero, read a guide about their skill build first.
It's fun to try out new heroes but too often people pick a hero and then just randomly pick skills without knowing what their hero can do, they might even skip core skills on a hero and end up playing really badly because of that, you pick Pudge for his hook because that's his biggest asset to the team, if you pick Clockwork or Potm you should have one point in every skill early on, so players of all levels should read about hero builds to make sure they aren't picking the wrong ones and costing their team.
3- Magic Stick is almost always a good item.
It's hard to recommend one item for all heroes but if there is one item I could do that for it would be Magic Stick. It's 200 gold and you can sell it later for half it's price so you're only really spending 100 gold in the long run, it takes one spot in your inventory and can provide a decent amount of burst heal and mana in a pinch.
Upgrading it to Wand is a bit tricky though as you'll be spending 300 more gold for 3 more stats and potentially 5 more charges, thing is you might never use the 5 extra charges or you might us magic Wand at 12, 13 charges at which point the extra 300 gold could have been better spent or something, so stick to Magic Stick for every hero.
4- Fill in the team gaps.
I often pick last whenever I solo que for a game, it's really been helpful because just by seeing what my allies pick I can find the best role for me.
Picking phase starts and Blue immediately picks Invoker, so that's our mid lane and I can't go mid anymore, 2 seconds pass by and Teal picks Weaver, that's our offlane most likely and that leaves me with either a carry or a support. Suddenly Purple picks Furion and says he will jungle, well I'm down to two options now and before I can pick anything Yellow takes Anti-Mage.
Now I have a choice, I can take Ember Spirit if I want to, but then I'd be competing for farm in every single area of the map OR! I could take someone like Rubick or Skywrath Mage and support my team, it's very annoying to always be the support but sometimes that's how Dota is, you can take a role you want and be selfish ( and most likely lose ) or you can learn to be a team player and fill in whatever role is currently unoccupied, it's usually support but hey someone has to do it right?
5- Master the basics
Last hitting, Denying, Pulling, Stacking and Warding are all essential skills to learn in Dota no matter what the role you're playing everyone should learn these skills.
If you're a support sometimes you'll find yourself alone in the lane, maybe a team fight just happened and everyone died, or your allies teleported away to another lane, the point is you're alone and you need to take advantage of that fact by getting as much gold out of your lane as possible. Same if you're a carry and you should know how to stack creeps so that you may take them later.
TL:DR version of pulling and stacking?
Hit creeps at the xx:14 and xx:44 seconds to pull, hit them at the xx:53 to stack. Only the below camp cannot be stacked due to the healer staying put if anyone gets attacked.
6- Adjust your item build.
A lot of players tend to fall into a pattern of buying the same item build over and over again if it works well in one game, there isn't anything really wrong with that but if you want to improve your chances of winning you need to maximise every single bit of gold you get and that means picking the best item build to fit in that particular situation.
If you're facing an Omniknight then getting a Diffusal is suddenly a very viable option for every hero, if Omniknight is on your team then you can consider skipping Black King Bar and getting other items instead. Doom has multiple single target spells so it's often better to get Linken's sphere against him rather than BKB, if you're playing Clockwork and farming a lot you might consider buying a Mekansm for your team if your supports have been dying a lot.
The point is there is no one-size-fits-all solution, sure you can make an item build that works 70% of the games you play but every once in a while you'll have to adjust your items to fit the situation you're in and the enemies you're facing.
7- Never underestimate early detection.
For some insane reason whenever people play against Rikimaru, Clinkz and Nyx they seem to either forget or choose to ignore that the enemy hero can be permanently invisible, I'm not sure why a lot of Dota players just assume that even though the enemy will almost always be invisible it's not a problem, it's a prety huge deal actually.
Too often players (supports actually) will simply refuse to buy dusts, sentries and later on a gem to counter these invisible heroes despite how easy they are to buy and how much of a difference they make, Broodmother is amazing in lane thanks to her spin web but what if you take that away from her? suddenly she just has her nuke and that's pretty much it. Rikimaru's ultimate provides just invisibility and nothing else which sounds so weak if you counter it doesn't it? get a gem and you nullify his ultimate completely, it's like the hero loses his strongest skill, for the price of one dust or gem.
Whenever I see an enemy Clinkz I just get a gem, doesn't matter how early in the game it is I get one before the 10 minute mark, why wait? sure dropping it is risky but it is far riskier to give invisible heroes free reign and farm than it is to hold a gem and Possibly die at some point in the game.