First, you'll need to know about how the guitar works. Some quick ones:
1. Guitar has 6 strings. Plucked with 5 fingers on the right hand - which also means care must be taken into sequencing since we use all 10 fingers to play the keyboard. To "force" myself not to overuse notes when playing, I use only 5 fingers as well when sequencing - 2 on the left hand and 3 on the right hand. Playing a difficult sequence involving all 10 fingers will be technically impossible on a guitar and therefore sound unrealistic.
2. Like all string instruments, guitar has positions. You'll need to roughly know them. I try to play/sequence the notes within a certain range, usually limited to 2 or 2.5 octaves. Of course, you need to know that there are open strings.
3. Remember the range of the guitar - don't go lower than E1 (in normal circumstances - not talking about alternate tunings).
4. Strumming can never be simulated well on a keyboard, unless with VERY good strumming samples. So unless you have great strumming samples (that includes up and down strokes, different speeds, short and long samples, choking etc), don't even try it or it will just sound fake.
5. Don't use sustain pedal. Hold the notes with your fingers. This will prevent unnatural ringing of notes when they are supposed to be played on the same string. You'll need to practice to know when to hold a note and when not to hold.
6. Use fret noises sparingly. Overuse just destroys it. Fret noises are produced when the fingers change position. So I'll add that in when position are changed, usually when I release my fingers from a sustained position. Roland makes it easier on their XV series by including fret noises as a release sample (and I believe only on heavier velocities) - it's easier to play but then still unnatural although it can fool an untrained ear.
I'm sure Iansoh and others will have other suggestions. This are just some quick ones that I can pull out.