DO NOT DIE!
For starters, the most important rule some tend to forget when action gets stiff: DO NOT DIE! It's ok if you don't kill as long as you don't die too (the final result of 1-0 is much better than say 5-7).
It may seem hard in the beginning (especially in -ar games) so if you're not very familiar with the game yet just run away when your hero is in red and go heal (if you consider yourself slow or the pressure is big it's better to start running in orange).
The only exception for this rule is when your friend needs help and you're able to give it. This is for more experienced players though since those of less skill tend to get killed along with their teammates which is very bad.
Now the second most important part: hero choice.
Most beginners after few games think that most powerful heroes are:
* Lich
* Venomancer
* Crystal Maiden
* Lion
* Slayer
You can't be more wrong, those are the weakest heroes late game, very easy to counter mid game and moderately easy to avoid early game (more on this in tactics).
When picking a hero try get your hands on some melee hero (preferably with str as primary attribute). Yes, they are the hardest to master and play but they pwn late game and by playing them you'll learn faster (after playing melee early, later on when you get ranged hero it'll be extremely easy for you to play it).
Some good choices:
* Slithern guard - my personal favourite, 2 disablers, speed increase for running away or chasing wounded foe
* Sven - needless to say this guy deals the most damage in the game
* Leoric - good for beginners, easily countered by magina though
* Naix - supreme pwnzor, very hard to play early game, requires different build
Another important issue: items
This will be hard to explain since most heroes require different items to work at their best. What I usually buy for melee hero is:
* Boots of speed
* Ring of health
* Sange (only if hero hasn't got some orb effect himself)
* Power treads
From now on things differ for matchup and hero.
If enemies have many casters go for black king bar instead of sange. After buying your first weapon go for some defence, blademail (good vs sven and melee), aegis of the immortal (just pwns but awfully expensive), heart of torrasque etc.and then more weapons.
Do not buy stuff you won't use later in the game (ring of regeneration, etc. - unless of course you're playing a hero that requires to do so - Naix and Magina for instance) it's waste of cash, go straight for the goodies. As an example I'll give here the list of items I had in my last game (played slithern guard) power treads, sange & jasha, aegis of the immortal, blademail, cranium basher, burize do-kyanon
You'll have to experiment a bit with items to see how it works.
Now some basic tactics:
Early game - try to stay out of the fight, just farm. When you're hurt go heal (better to lose some exp by not being there than lose cash, feed oponent and not be there too). Whenever you see more than one enemy going for you: run.
Countering casters - early game: nothing you can do about them, just try to avoid them. mid game: get black king bar, it makes all their skills useless. late game: laugh at them as they try to do something to you with their ~500dmg spells.
Countering melee - blademail, disablers (stunning is best, bashers work wonders, also sange with maim is good).
Countering invisible - those are real pain in the ass but simple gem rends them useless (if you think you'll die anyway then don't buy gem, go for wards).
Switching lines - something most beginners forget, when you see you can't deal with enemies on your line - switch with teammate who's got hero better suited for the job.
Gangbangs - you must remember that mid-late game it's all gangbangs, you'll rarely have a chance to fight 1on1. Just try to stay together with your team at all times and be ready to run at any sign of enemy.
It's all just a bunch of loose advice. If I forgot about anything and anyone would have any questions then ask them here and I'll try to answer them.
Good luck and have fun!
