I am a Turkish citizen who had an F-1 visa until the Summer of 2016, since I was a student in USA. Now I will need to travel to US for a conference and will apply for a B-1/B-2 visa soon.
Do I need to apply for DS-160 again or not?
DS-160 is the name for a US Nonimmigrant Visa application form. It's what you need to fill out (electronically) to apply for a visa (unless you're immigrating to the US and are applying for an immigrant visa, at which point you need a DS-260 instead). You'll need to submit one (online) every time you apply for a visa.
As noted in the FAQ, it is possible to retrieve some of your personal information so you don't need to retype all the bits that haven't changed, though you are still responsible for ensuring that all information is correct and up-to-date:
I apply for U.S. visas often. Can I reuse the DS-160?
Yes, you can use information from a previously submitted DS-160 to populate some fields on a new form. There are two ways that you can do this. First, if you plan to apply for a visa in the future, save your DS-160 to your hard drive or a disk using the instructions in the FAQ above. When you go to apply for your new visa you can then select “Option B-Upload a Previously Saved Application” on the Getting Started page. Alternately, if your previous visa application was submitted after November 1, 2010, you can select “Option C-Retrieve Application,” enter your previous visa application ID, and then hit “Create a New Application.” Your personal information will then populate the form. Be careful to check that all previously submitted information is current and accurate.