When you begin to pick single notes or strings on the guitar you should begin with the downstroke. Downstroke picking is much like strumming. Most of the strumming rules also apply to downstroke single note picking. If you haven't gone over strumming make sure to check out the basic lessons on those.
The big difference between strumming and a single downstroke is the need for accuracy. Learning to control which string you pick is more challenging and takes better hand-eye coordination.
Whether you are reading tab or standard notation, there are a couple of ways you can notate a downstroke. If none of the symbols below are present on your music then how you pick the note is left up to you. Generally if you follow the rule of downstrokes on the beat and use your picking hand as a pseudo metronome you'll probably be playing in the most economical way.
Downstrokes can be indicated by either an arrow pointing downward on the paper or the following symbol.
Picking each open string is a good way to focus on your picking without having to worry about what your fret hand is doing. This simple example shows four downstrokes on the low four strings of the guitar.
Once you have mastered picking each string successfully, you can focus on muting or dampening the previous open string as you move to the next open string. This is either done with the fret hand lightly touching the string that shouldn't sound anymore or using the palm of your picking hand to mute or dampen strings. Good players will use a combination of these two techniques to deaden strings that shouldn't be heard. Even though the previous example looks easy to play, dampening the notes when appropriate add a degree of difficulty when you are learning this technique.
Playing through a scale will allow you to work your single string picking, involve your fret hand and sound somewhat musical at the same time. The example below is the A minor pentatonic scale. This is just a sample but give you an idea of how playing through a scale will really help to build hand to hand coordination.
Below is the scale diagram of the A minor pentatonic if you prefer to read scales this way over the tab way. Always use a metronome and make sure (by watching your picking hand) that every stroke is a downstroke. No upstrokes...yet.