I have no prior experience with any DotA type game, or any type of strategy/RTS game whatsoever, yet this game has drawn me in with it's awesome character designs and art style, and I figured it was as good of place as any to start playing this type of game. 
 
So, any tips so that I'm not a complete abysmal failure?

tip #1: don't say "n00b".  It makes you look like a noob.

Patience! Playing defensively in the beginning is the key.

Don't be afraid to retreat.  If you are near death just run, you will avoid waiting to respawn and will keep your death count to a minimum.

Here are some things that helped me when I started:
 
1. Get some kind of health  item (or potion) at the beginning. 
2. Teleport and Heal are good summoner spells to use. Teleport will allow you to return to your lane quickly so you dont miss out on gold or xp and to help defend. Heal can save you from a gank early game.
3. It helps to stay somewhat in a group with your other teammates mid to late game if you find yourself getting ganked a lot.

The tutorial's pretty good, it points out some basic ideas such as using your allied units to tank damage and whatnot. And if you still can't quite get it, you can make some bot matches to practice until you have some basics down.

This game hates me but that is OK since i hate it right back.
 
One thing that will come in handy for you is to hide in the high grass with another player and ambush ennemy players once the passed your position.

1. Don't be greedy. You are more useful alive than dead, even with +1 kill. Especially in later parts of the game.
 
2. Try to pay attention to your minimap. Try to see what your team is doing and work with them. The most coordinated team wins. Also pay attention to how many enemies are visible. If one or more are missing, be prepared for a gank attempt and don't push out far without at least as many teammates.
 
3. Play matchmaking after you understand the absolute basics. I think you will have more fun/learn more than playing practice games.
 
4. Health always wins early game. Get a health item or at least a couple health pots. Try to figure out if their team is unbalanced towards magic or physical damage and compensate by buying a countering resist item. Remember, though, that resist items are much less valuable if you have a small health pool. Get Mercury's Treads if they have a lot of stuns. A big mistake I see new players making is never buying any defensive items. Don't go overboard, but a good balance makes a huge difference.
 
5. Don't worry about learning the specifics of every champion or about playing every one. Find one you enjoy and then try to get good with it. All you need to learn about other champions is, “what can this champion do to me in x situation?” Just pay attention to what your enemies are doing as you're playing and you'll learn what you need to know about all the champions eventually (be grateful, there are about three times as many heroes in DotA). 
 
I think that casters might be more difficult to play for newer players, so look at tanks and physical damage champions.
 
There are a bunch of these threads on the LoL forums if you look or search in the Guides/Strategy section, as well.

" 1. Don't be greedy. You are more useful alive than dead, even with +1 kill. Especially in later parts of the game.   

It's all about knowing the items, learn them inside and out.
 
DoTA (therefore LoL) players are ****ing assholes. When I started playing I always got treated like shit, so brace yourself.

Stop feeding the other team's carry! :p  
 
Seriously, though, it's best to just play this game with 4 other friends and get a gewd team synergy going.  That's probably like 50% of kicking ass in this game.