Kenji, if you are interested in attaining theta, you should read the book "Awakening the Mind" by Anna Wise.
Brainwave states aren't necessarily as simple as they sound. The first thing to understand is that right now, you have brainwaves resonating in all 4 states. Most are probably beta, but that doesn't mean you don't have brainwaves in Alpha, Theta, and Delta as well. Also, these 4 states are merely general categories for the hertz of your brainwaves. So having most of your brainwaves at around 8 hz, you would technically be in Theta, but a much lighter theta than if most of your brainwaves were at 4 hz, which is probably the theta you are referring to if you are talking about restructuring your subconscious.
To do any "work" while in theta, it's generally a good idea to be conscious of it. This means you will also need to have a certain amount of BETA brainwaves active during your time in theta. For example, you pass through low theta every night before you fall asleep, but probably don't remember it. If you've ever had pre-sleep visualizations (called "hypnagogic" dreams) that's a good example of how you pass through low theta before hitting sleep.
If you want to experiment in the theta state without having to practice meditation(although I highly recommend it) or buy an EEG(which will really show you how complex all of this really is!) I suggest the following:
1. Capitalize on the time spent inbetween sleeping and waking, and experiment with the hypnagogic state. This can be done both after just waking up, or right before you fall asleep.
2. Learn to lucid dream. Tests have shown that while lucid dreaming, both Theta and Beta are very active. I highly recommend Stephen Laberge's book "Exploring the World of Lucid Dreaming"
3. Self-Hypnosis is a method of reaching low theta. I haven't tried this myself, but some have reported success with it. There is a wealth of information available on the internet and elsewhere.
4. Drop some acid, eat some shrooms, and chomp some peyote. No, I'm kidding, I don't actually recommend this, but it is the basis of much interesting work done in many native south american cultures, as any Castaneda reader can tell you
Good luck, and I'll be happy to answer any questions.
-Dylan