The first thing you need to do is tune your instrument. I recommend the Fender Tune app for guitar or ukulele. It is free and easy for beginners to use.
You can also check the links below.
Listen to each string. Turn the knob until your string sounds the same.
This has free step by step lessons for learning guitar chords.
This has free beginner ukulele lessons. This teaches chords and strum patterns.