I have a feeling there is more to your question than your saying.
You can make your robot avoid objects but to make it run a strait line you need to be able to sense the position - with a compass, or an accelerometer or by monitoring the rotation of each wheel. Your SR04 isn't going to help you drive a direct line.
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So, how could be the code to make my robot run strait using an encoder?Using PID, I'm searching things about PID but I until don't found a site that explained me it.
Don't need that to make a robot drive straight. You need that if you want the robot to see objects in front of it so it can either stop or avoid the object.