Practice playing individual notes. Holding down a string and producing a decent sound can sometimes be more challenging than it looks. If you don't hold down a string hard enough, you'll get a muted note and if you hold down the string too close to the fret your guitar will buzz. Practice picking in an up and down motion on your string with the other hand. Continue doing this until you feel comfortable moving up or down the neck to a different note. Practice playing the notes back and forth until you become comfortable strumming.
Learn how to transition to different chords. One way to become proficient quickly is to learn how to transition between the various chords quickly. These transition periods are often the hardest for beginners to play when they are starting off. The more you practice between switching to different chords, however, the better you'll get at doing it in songs. Practice switching between open chords like G, A, E, and C. 
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