I have the latest version of photoshop and two images.
One of me (very good quality) and one of a patio type area (pretty good quality) . I cropped me out and pasted it onto the patio but it looks fake like a cut out. I am sitting cross legged and it looks like I am hovering in the air even though I am clearly on floor. How do I make it look real?
You'll need to fake the lighting on you to match that which lit the patio scene, and fake the shadows which you would have cast onto the floor, etc, had you been in the scene. There's also indirect lighting (light being bounced from surface to surface, like between blue jeans and a wooden patio) to consider.
There must be hundreds of tutorials that can be found through Google. I realise that the difficulty is identifying the good tutorials amid the mountain of rubbish but I'm sure you'll find some. Don't expect to produce great results without a significant amount of practice. There's much more to creating convincing fake photos than adjusting brightness and contrast. You need to consider the direction, colour and intensity of light sources which illuminated the scene (the patio) then make the subject (you) appear to have been lit with these sources instead of the illumination when the photo of the subject was captured, and simulate the interactions (shadows and indirect illumination) between the subject and the scene.
Assuming you have your subject in a separate layer above the background, it's often effective to use the Burn Tool on the background with a fuzzy brush and low Exposure value to subtly darken the ground under the subject.
Not too long ago I was at an airport with a group and we wanted a photo of us all together. There was no one around who could help us, so two of us took turns shooting while everyone else stood still. When I cut him out of the photo I took to paste him into the photo he took, I had to do just what I'm describing here, faking some shadowing and even subtle reflections in the floor (it was waxed and a bit shiny) to make it look like he was really standing there.