Similar to the violin, the cello has four strings and they all need to be tuned in perfect fifths. In order of thickness, from the least to the thickest, the strings are C, G, D, and A. Many students use a piano to tune their C string because it corresponds to two octaves below middle C on the piano.
However, beginners should use a digital tuner at first. Start tuning each string separately from C to A by slowly tightening the strings and looking on what the digital tuner displays. Don’t perfectly tune a string right from the start because that will cause uneven pressure on the bridge and you will affect the other strings.