So let us look at some Design/Prototyping tools:

Sketch 3 (Bohemian Coding)

Sketch 3 is a Vector based design application. It’s easy to use, and has a low learning curve. Think of a user friendly mix of Photoshop and Illustrator
You have the possibility to use a ton of plugins to make the design experience even greater.
And using the template components for Material Design should make designing great applications fun.

Price: $99
Makes me happy because:

• Easy to use, no high learning curve
• Support plugins
• Many templates are available

Makes me sad because:

• Updates can break created designs
• So support for live viewing on an actual Android device

Illustrator (Adobe)

Adobe Illustrator is also a Vector based design application. It contains a lot of features for the skilled designer. But if you are a Illustrator newbie, you can be overwhelmed by all the possibilities and options available.
As with Sketch 3 they also have a template components for Material Design which you can use.

Price: €24.19 per month for just Illustrator (or €69.49 per month for entire Creative Cloud suite)
Makes me happy because:

• Focus on screen design
• Many templates are available

Makes me sad because:

• Learning curve is to high
• So expensive

Flinto

Flinto is designed as a tool for creating interactive prototypes based on PNG’s. Just link your PNG’s that you have created to screens and set transitions for that app-like feeling.

Price: €20 per month
Makes me happy because:

• Ease of use for prototyping
• Great transitions

Makes me sad because:

• You can only use it for prototyping
• Kind of pricey

Marvelapp

Marvelapp links your prototype with Dropbox to create an interactive prototype on your mobile device or desktop. No need to code or save PNG’s as images.
It’s easy to use, and can be used with different Vector based design applications. You can even export your prototype to a runnable APK.

Price: starts from €0 per month to €100 per month.
Makes me happy because:

• Ease of use for prototyping
• Great UI
• Exportable APK’s

Makes me sad because:

• No unlimited exportable APK’s

Keynotopia

If you know how to use Keynote or Powerpoint, you may want to use Keynotopia. Keynotopia is a library that contains UI components.
So you don’t need to know how to use a different design tool, just drag en drop your UI components to make your design.

Price: €49 if you are just interested in Android design.
Makes me happy because:

• No need to learn a new tool
• Free life time updates

Makes me sad because:

• UI components aren’t updated regularly
• Still no Android preview app available

Pen and paper

Last but not least, you can just you a pen and a piece of paper to do your design. It’s doesn’t need to be a paid app or service to start designing awesome designs.

Price: €1.
Makes me happy because:

• No limitations

Also available:
InVision, Proto.io, POP, UXPin, FieldTest

finally let me give you one advice, if you have no previous graphic design experience then I would suggest that you mastering the basics of graphic design first.
Once you are comfortable with the basics then you can start to look into designing with a specific tool.