Play Nevermore, he requires a bit of skill to use, regarding raze placement, the distance, chain-razing, using lothars to position your model "inside" an enemy hero, ulting, and watching clinkz rage after you kill him in 1 blast.
As far as ranged heroes go, I am a fan of Vengeful Spirit(good dmg, good stun, useful ulti). Also if you get good with character pathing Drow Ranger can be deadly. Recently I have been into melee though and have been rocking characters like Kunkka, Sven, Bradwarden, Slark, ect. Also if you want to get some good practice in and can't find anyone to play with grab one of the AI maps.... Insane AI is pretty decent practice.
Edit: If your having trouble moving to melee try Skeleton King, he is pretty noob friendly, and generally pretty awesome.
I don't have much experience in strength/melee heroes in particular, I find it difficult to take the enemy's hits. I've tried Dragon Knight, which was quite nice with decent starting stats, but I was highly susceptible to harrassment by enemy players even with movement and strength boosting items.
Can anyone give me a general guideline to the strength class?
The strengh class is basically a tank. That's the best thing they do. They go ahead of everyone, stun an enemy hero, and beat it to death, while also taking the damage, while the rest of the team(DPS-ers mainly) aid him.
Therefore, tanks are high HP and Armor heroes, with spells that stun, so make sure you know which hero has stun spells, if they're area damage type or not(these are the most preffered), and so on.
Resonable build on any tank begins with the Ring Of Health, turning it into a Vanguard, then make sure you build your Black King Bar, it's Avatar ability turns you invincible to any spells for its duration(Make sure you activate it, I constantly forget to do that part). Then I'd suggest the Battle Fury, to help you farm, and then it's up to what your team's needs are and what you're against that you should base your build on.(Don't forget your boots).
IMO, the best tank out there is Tiny. Nothing beats that thing if played right. It has two stun spells, one active(also its area-type, so it rocks) and one is passive, although you'll want to improve on that one after you got your fist two spells to max).
Worst tank would be Huskar, but if you don't use it as a tank, it turns into a ranged-strength hero, with lots of HP and a big dose of Agility. This is my favourite hero. Just stay away from spell spammers in early game.
If you want some more pointers and stuff, hit me on skype.
You can find a lot of guides on certain websites that i dont know if i can mention them here but : dota strategy and playdota. they have a lot of general guides like game mechanics and a lot of good individual hero guides. you can find good item builds for certain heroes there, aswell as good strategies on how to play that hero.