To transparent the image you have to choose the right layer you want to make transparent. If you want to make the backgrouond transparent (the first layer) you have to copy the background to an own layer, and clear the background. On Layers you have a parameter Opacity you can change from 0% to 100%
About using brushes: Uh! I tought myself using the brushes for drawing and painting, so I don't really know good tutorials. Also I found out that SAI is much easier to use for painting, so I only use photoship as an additional program.
But you will find a neverending bunsh of tutorials here: Browse Art --> Category: Ressources and Stock Images --> Tutorials --> Digital Art --> Drawing and Painting --> Photoshop [link]
It can be hard to find the right tutorial, so you can also take a look at #TutorialsClub and other groups like this.
helpby #ArtChance