If I chose a party do I have to vote for all of their candidates?
It depends on the election. In primary elections, you can only vote for candidates from the party to which you belong. In general elections, you are free to vote for anyone you like, regardless of their party affiliation.
Can an independent vote in a primary election?
Yes and No. In Iowa, you must be a member of a political party (currently: Republican, Democratic, and Libertarian) to vote in a primary election. However, anyone* can change parties on Election Day and vote. For independents to vote they must declare a political party.
*Precinct election officials may not change political parties within 30 days of an election.