ok when you first go to onderon/dxun and you meet the blockade..what exactly happens? i know you get shot at and start a great battle but i dont get it...someone enlighten me plz?

Its just a plot device.

 

By having the battle start and you being forced to land on Dxun instead of Onderon, you are forced into the situation of helping Mandalore and investigating your past (as Kreia says, Dxun is where the Mandalorian War began, the Exile fought there).

To expand on it a little bit:

 

Onderon and the Republic have been at odds for a while, so if I understand it right, the Republic sent a peacekeeping fleet to Onderon to make sure they stay in line. That doesn't sit well with the Onderonians, who keep their own fleet close to the Republic's to show them they aren't going to take their guff.

 

Certain factions within the Onderon government have powerful allies that want you captured (I don't know how far you are so I don't want to spoil it), so as soon as the Ebon Hawk jumps into the system they detect it. They move to try and capture you, but since the Ebon Hawk's registered freighter the Republic fleet must've taken it as some sign of aggression and started firing blindly.

 

The space battle that erupts would probably be the result of fog of war and no one knowing what's going on.

ok thx