The most common scale used in lead guitar playing is the minor pentatonic scale. Pentatonic means five tones (notes). In the A minor pentatonic scale these notes would be A C D E and G. The following diagram shows the A minor pentatonic scale in the 5th position.
Play the scale as indicated here. It may sound familiar to you.
Now try starting at the highest note and playing down through the scale.
The next step is to play the scale ascending and descending as shown here. Try to get a smooth, even sound.
To help develop your picking it is a good idea to practice playing each note twice.
Riffs are often derived from scales. This one uses notes from the A minor pentatonic scale.