He used the term frets caz he`s a guitarist. Frets are something like yr fingerboards...(TO Sean)
Well, regarding to how much yr playing of the guitar will help u. U`ve got a slight advantage, as u play the guitar...so I`m guessing that u have pretty big and strong hands. This`ll surely come in handy, as the cello strings get harder and harder to press down the position. And a strong hand will result in a good vibrato, that is if u do it the correct way. There are many complications involved when playing a string instrument. So, as to what is the most defficult thing about playing the cello, well, I can assure u, alot of things are difficult. Unless yr deciding just to play for fun , and not give a damn abt technic. My advice is that u get a teacher and learned it in the serious and proper manner, if not, u`ll not be doing justice to such a wonderful instrument. Okie, here`s what I`ve noticed after teaching for some time.
1. A good bow hold is difficult. But it`s essential to get it correct, caz without this, u can`t get anything done.
2. A good bite on the strings is difficult.
3. Trying to not move the elbow too much is difficult.
4. Changing strings and keeping the `central balance` is difficult.
5. Moving positions is difficult.
6. Vibrato is difficult. Unless u`ve got pretty good muscles co-ordination. And that they`re relaxed, not tensed.
Well, but all these would soon be nothing, once u start yr lesson and take it seriously. Go find yrself a gd teacher. U won`t regret this! Good Luck!