Hard to understand becoming in paticca samupadda It is said that dependent on becoming birth arises. 

How can I understand becoming? What is becoming? Can you give me a simile by which I can understand it?

For example it is very easy to understand that dependent on birth, death arises because these two concepts are like the two faces of the same coin. You can't have one without the other.
How to see becoming to make it clear?
In Theravada Buddhism, there are two kinds of bhava:

kamma-bhava - the becoming that is kamma
upapatti-bhava - the becoming that is coming-to-be

The former is what is directly understood to be referred to in the teaching on dependent origination. It refers to the wholesome or unwholesome mental formation that constitutes volitional activity. It is the result of clinging, since clinging leads directly to acting on one's intense desire.
The second type of becoming is a by-product of kamma-bhava and is actually more or less synonymous with rebirth.

As it is said, &quot;Bhava paccaya jāti,&quot; rebirth occurs in the human and deva worlds or in the lower worlds because of good or evil kamma-process. So rebirth stems from kammas which result from clinging (upādāna) and craving that is rooted in the contact between the six sense-objects and the corresponding sense-organs (āyatana).
In other words, there arise viññāna, nāmarūpa, salhāyatana, phassa and vedanā in the present life as the avijjā saṅkhāra, etc in a previous existence and now on top of that, taṇhā and upādāna give rise to new rebirth. The situation is like that of a man who has committed a crime while he is in prison for a previous conviction, or that of a man who has incurred new debt before he has fully settled his old debt.
Such new kammas accumulate by the thousands in a single lifetime. Under certain conditions one of these kammas becomes a death-bed vision and leads to rebirth while other kammas will create rebirth at other times in the life-cycle. If there are residual kammas from the previous lives that possess great force, they take precedence over present kamma, appear as death-bed visions and create rebirth in the lower or higher worlds. The post-mortem destiny of the person in such cases is determined by the nature of kamma.
-- Mahasi Sayadaw, Paticcasamuppada

As a simile, upadana is like the heat of rubbing two sticks together, bhava is like the igniting of the spark, and jati is like the flame and smoke that arise based on the ignition.