Best thing to do is to use the plain old lasso tool (l) to quickly and roughly outline the figure you want to transplant. Drag it into the target image with the move tool (v) and scale and reposition with the transform tool functions
apple: + t). Hit return when happy.
Next add a layer mask to the layer of the person you just added. This will allow you to paint the layer's opacity over the background layer.
Hit 'd' to reset your colours to black and white and switch to the brush tool (b). Painting in the layer mask (make sure you've got the layer mask selected and not the colour part of the layer) with black will hide, while painting with white will unhide the pixels of that layer.
Use hard edged or soft feathered brushes to get the desired type of edge blend. Don't worry about making mistakes. You're not erasing pixels, you're just hiding them. If you accidentally painted out a bit you want to keep, switch colours to white (x) and paint them back in.
Play with the Opacity and Flow percentages of the brush tool to control the behaviour of the paint laid down by the brush tool. [ And ] will scale the brush size down and up respectively.
Add adjustment layers grouped to the added person layer to do any necesarry colour grading and luminosity balancing. Adjustment layers can always be tweaked, removed or blended, so unlike the normal grading tools they are lossless. Keep an eye out for black levels, try and match them as closely as possible for better results.
If you really want to go to town you can have a go at fixing any light direction differences that might be present but that's perhaps a step too far if you are a beginner.
The more precise you are, the better the results will be, ultimately.
Save document in PSD to preserve all your work.
Hope this helps!