My natural modesty (the best of my many admirable qualities) made me wait for someone else to come in on this, but I guess all the really experienced Doms are too busy saving the world, curing AIDS or pleasuring their vast harems to have got around to it, so you'll have to make do with me.
Well done for having the courage to ask, and good luck on your journey into this wonderful world. Don't be coy about questions that sound too basic, the only silly question is the one you don't ask.
To start with the one you've specified, subspace is used for a range of states from intense overwhelming submissive feelings, to something like a trance or out of body experience. It's really the end of a road you've just started, so don't feel you have to get there right away.
The way most subs first reach it is lengthy SM in tight bondage; intense pain needn't be involved, it can involve low-key sensations such as scratching or pinwheels, the key thing is to deepen the sense of being a helpless object till the sub loses herself in it as a meditator loses hirself in the mantra. Blindfolds and other sensory shutoffs help. Like orgasm, it gets easier to reach with experience and can be linked to particular triggers: a sub on this forum says that her Master can send her there just by gripping her neck.
You might get more precise answers on this particular question by posting it on the Submissives' side. Meanwhile, what are some of the other issues?