>>5061
>Should I actually look at the thing or should I just try to imagine it?
What have you chosen as your object of concentration? Personally, I like to concentrate on the breath. There can be some mental imagery of the breathing, and sometimes that helps, but usually my practice focuses on the experience of breathing, the movements, any sounds, etc.
>Should I try imagining it outside or inside me? (If that makes sense?)
Again, this depends on what you are concentration on. Are you using a mental formation, like a mandala? Otherwise, just follow your breath with your attention, watch with your minds eye, that is. No need to imagine anything, just pay attention to whatever sensations you are getting from your own breathing (if following the breath). Just return to the object of concentration any time you've noticed you've lost it.
>Should I do it for a set time (i.e., trying for 10,20,30 minutes, etc. minutes), or until I fail (i.e., 10,20,30 seconds, etc.)?
You should "fail" about every 30 seconds or so, depending on your natural abilities. Note that it is not failure to catch yourself in thought, it just is. Notice it, return to the breath.
There is a direct correlation to how much and how hard you meditate and the gains you will see from it, just like weight lifting. However, burnout is also very common if you push too hard too soon. I would suggest a daily session, starting at 5 minutes, and do that until it is too comfortable and you naturally want to sit 10. Arbitrary goal: six months from now, a 15 minute session should be nothing for you, and a 30 minute session should be a nice challenge. A 45 minute sit should be scary/exciting.
Personally, I sit two 15 minute sessions a day, and one 30 minute session. My local Zendo sits 45 minutes at a time in their shortest session, if that gives you any kind of ball park. There is a sweet spot where you get into it, so eventually a 5 or 10 minute sit just isn't long enough. However, it is also the case that if you meditate throughout the day it becomes an ongoing process, even while sleeping.
>DO I have to do the other limbs too?
No, absolutely not. It is quite common that people only do one branch, like the yoga bunnies who only do asana, or the Zen folks who only really do the last three limbs and don't emphasize physical postures beyond the lotus position.
However, I would urge you to at least investigate them, they all support each other. Your concentration will be better if you live a moral life (first two limbs). You'll be able to sit better and longer if you stretch your body and keep it limber (third limb). You'll concentrate better and have improved cognitive abilities if you can control your breathing (fourth limb). You will most definitely concentrate better if you can control the external facing senses (fifth limb). You are already interested in the attention and focus training of the sixth limb. If the religious aspects of the seventh and eighth limbs turn you off, by all means, avoid them. But man, you are missing out on some amazing bliss and joy my friend, and if you've worked on the first six limbs, the seventh and eighth might just come anyway, if you set the right conditions and prerequisites. :)
Namaste!