How to take pictures like a tumblr girl?

And i don't mean how they look/dress, but how they make their pictures professional(they look professional to me) i just bought a Nikon d5200 and i'm getting the hang of the settings and experimenting with it but i want to know exactly how these girls do it

A good camera is obviously the first step. Bloggers often use a 50 mm lens.
Apart from a professional camera + lens, it is really important to have the right lightning. The most flattering light is either in the morning or in the early evening (when the light is soft but still bright enough for pictures not to come out grainy).

A lot of those Tumblr accounts also use a lot of filters and photo editing to a large degree in order to achieve a particular effect. Try experimenting with filters with apps like Instagram, Afterlight, or PicStitch. It's fun and allows you to achieve virtually any look you want!