If you have a pickup in your guitar you can plug it directly into the tuner. The tuner has a normal guitar input jack that enables you to connect your guitar cable.
Most guitar tuners also have an extra jack allowing you to connect your tuner inline between your guitar and amplifier. Ensure that your guitar is plugged into the input jack and not the output jack that goes to the amplifier. Other things to consider when using an electronic tuner are:
Check your manual to see how to tune you low sixth string to the note D. Different tuners vary in operation. You may need to play the note on the 12th fret, sixth string, thus producing a note which will be the same as the open 4th string.