Basically, elements that make covalent bonds (like PO4) want to have a full outer shell, meaning that they will have 8 electrons. But, they can share electrons with other atoms. So, since phosphorus has 5 electrons in its outer shell, and oxygen has 6 electrons in its outer shell, an oxygen can be attached to the phosphorus so each atom with be surrounded with 8 electrons. But! There is an extra electron, so, it is taken out. That is why there is a positive 1 charge on po4.
