<font face="Verdana, Arial" size="2"> But we also have to hold the structure of our body. How do we do that without using any force? </font>
Hold the structure of the body, but relax inside the structure. Eventually, the stiff strength and tension inside will dissolve and pung-jin will strengthen. Like the balloon analogy, I think the chi inside will be like air in a balloon--moving freely, but supporting the outside structure.
<font face="Verdana, Arial" size="2">I asked someone once, and he say that our structure is hold up by peng-jin. every posture/move in taichi must have peng-jin.</font>
<font face="Verdana, Arial" size="2"> He also said that peng-jin is like rubber balloon that does not matter where the force is apply it will not move (or something like that) This even confuse me further because how can I do the form in relaxing manner how can I resist the force that is apply to me.</font>
Maybe think of a basketball. The air inside holds the ball so that the surface is firm. The air inside is soft and relaxed. It has no tension but it creates an internal pressure. When force is applied by forcing the ball against the ground it does not collapse. The ball just bounces. When the body is really relaxed, extended, and open, then the body can become more like a ball--springy and distributing force evenly.
Yet the ball is should still be very responsive--it moves according to where it is moved by the player but it always maintains its structural integrity as a ball filled with air.
<font face="Verdana, Arial" size="2">So my question would be....Do I continue to do the form in a completely relaxing manner(which I still couldn't) and this peng-jin would develop eventually?</font>
<font face="Verdana, Arial" size="2">This would mean than at this time I still do not have the internal streght to resist the force without using muscular strenght? </font>
Maybe. Everybody has a certain amount of internal strength. It's not so much a question of enough internal strength. The problem is more that internal strength gets stuck when there is still too much tension in the body. It's a natural tendency to respond with muscular strength too, so even when people learn to relax a little and use internal strength in their form they often resort/revert to using stiff muscular strength in push hands.
Don't worry about it too much, my advice is 1) maintain the outer shape of your form (don't be limp) 2) relax inside the form. It can take a long time--but it's worth it!