The same way you would "drag and drop" songs onto the itunes is the same way you would import them into your iTunes.
I know it may seem hard at first, but it's better than using some third party software that may cause your phone to "brick" or void your warranty.
To import songs into iTunes, just select/highlight the songs that you want, drag them to the top left corner of the iTunes window where it says "Library," a green circle with a plus symbol should appear near your cursor, then just drop the songs and they should import automatically.
When you insert your iPhone, click on it when it appears under "Devices," then in the middle of the iTunes page should be tabs, look for the one that says "Music," click on this tab and choose "Automatically fill" or "Select All Songs/Playlists," and it will automatically fill your iPhone with as many as it can fit.
