Yoga by itself doesn't make clear what one is doing per se and for what really, not in totality at least. Thats why yoga is said to go hand-in hand with sAMkhyA, which is the theory behind yoga practice. The basics of sAMkhyA tell one that he is the purusha though he finds himself stuck in prakRRiti. The path of yoga is a practice that delivers one's freedom as purusha. I've always stuck to my opinion that Yoga must go through all the 8 steps of aShTAnga yoga. There is no skipping levels and jumping straight to yogAsana & prANAyAma if the goal is indeed meditation. These two steps are just good for physical health otherwise and somewhat mental, and the resultant benefit is just worldly without yama & niyama.
"When a yogin becomes qualified by practicing yama and niyama, then the yogin can proceed
to Asana and the other means." --Yoga Bhashya Vivarana (II.29)
yama and niyama lay the foundation for one to focus on the Asana positions well and the movement of prANa during prANAyAma. yama is made up of satya (truthfulness), ahimsA (non-violence), asteya (non-stealing), brahmacarya (celibacy) & aparigraha (non-covetousness), while sauca (cleanliness), santoSha (contentment), tapas (austerity), svAdhyAya (self-study), Ishvara praNidhAna (surrender to God) make up the niyama. One must try to follow these to the best of one's abilities. As one becomes well-established in these basic rules, over a period of time, his citta is shuddha and then he can build over these while carrying on to yoga and prANAyAma next.
The basic idea of Asana is to keep the body flexible enough to maintain a position without physical or mental agony over an extended period. Before reaching any meditative exercise, one has to be able to disconnect from bodily suffering, else what kind of "meditation" would that be? Even prANAyAma would not give good benefits soon if the Asanas don't make prANa movement flexible. Most AsanAs are to be done in stages of sync with breath. A classic example is suryanamaskAra that has inhale, exhale and breath retention tied-in per stage. Once the body is flexible enough to perform the AsanAs painlessly and the movement of prANa through the body is smooth, one can proceed to maintaining one of the sitting postures for longer period of time.
While the practice of the above continues, one should move onto a very important stage before the doorway to meditation: pratyAhAra. This is also the least understood anga. All the yama, niyama virtues gained are good for meditation only if pratyAhAra succeeds, because this stage links the bahiranga (external) stages with the antaranga (internal) stages. A great analogy for what pratyAhara means is what a tortoise does. The way a tortoise withdraws its limbs within its shell, so should one withdraw his attention from all the five senses. This is an extremely important step in sAdhana of a yogi. This makes or breaks a yogi, so to say. The mind is fed all the data from the senses, other than one's own memory to process. Pulling one's attention from these inputs lets a person use his mind for the goal. The practice of this step can continue with some senses even while in simple worldly activities.
With the above internalization, one becomes ready for dhAraNA (concentration) on an object, breath or IshTa devata, etc. When this dhAraNA towards the object becomes steady over a period of time, without any distractions whatsoever, it is said to be tailadhArAvat (like a steady flow of oil) and then dhyAna (meditation) happens! Its not something you do, its a happening! So when you say you can't meditate, its because you're trying to meditate, what you need to do instead is remove all obstacles that you think are troubling you. Be honest. Accept that there are troubles, else you would be realized already... why, you wouldn't even have been born in the first place! :) So be true to yourself, try to remove obstacles with all the bahiranga and antaranga steps, keep concentrating and meditation will happen.
I'll close this now, what was intended to be a short note, by saying that I'll purposely skip the last anga samAdhi which has no English word per se and its something that can best be left unexplained (till I someday write a series perhaps, if there is Guru icchA) and only be experienced.