Q:

Create a multicolor background

I'm a novice photoshop user, i stumbled upon the designs of the awesome mike of creativemints, and i noticed how his backgrounds are really beautiful and smooth, i tried to recreate it and failed miserably so my question is, how do you achieve such effect??
 
and thank you so much!!

A:

You can also use a very large and soft paint brush to paint big blobs of color on top of a dark blue background and then blur it to achieve a soft gradient look. In your example image, this is most likely the technique.

Set your background to dark blue
Use a large paint brush (Shortcut: B) with hardness set to  0% (you can make brushes small/larger using the [ and ] keys, and softer/harder using SHIFT + [ or ])
On the background layer, paint big blobs of pink, teal and alternate
green blues around the canvas
To achieve a very soft 'gradient' look, go to Filter > Blur > Gaussian Blur with the Radius set to around 50px.

Here's an image I created with this technique.

