they always ask me to do it. However, all of them commonly say that I cannot take nice photos

It's weird that they don't like your work but continue to ask you to do it. Maybe there's some teasing/flirting going on? It's great that you want to improve your photography, though. Keep it up.

There are a lot of conventions about what makes a good photo, and learning about that will certainly help, but your friends aren't evaluating your photos for technical merit. They probably care mostly about whether or not they look good: the better you make them look, the more they'll like your photos. With that in mind, here are a few tips:

• Get them looking at the camera. If they're looking, then they're less likely to have their eyes half closed, have some weird expression on their face, etc.

• Get close. Fill the frame with your subject(s). That'll help your composition, avoid the possibility of having something distracting in the background, and give them more of what they want to see. If you're traveling, then you'll probably want some shots with local attractions visible, but try to still make your friends the center of attention.

• Pay attention to light. Specifically, pay attention to how the light makes your subject(s) look. Is the direct sun causing dark shadows on their faces, or making them squint? Is there a bright background that's making the camera underexpose your subjects? Just looking at your photo with light in mind will help you take better shots. People tend to look better in soft light, i.e. light that comes from lots of different angles, so shooting with your subjects in the shade of a building with open sky above is a good strategy. Shooting on a cloudy day is good for the same reason. If you're indoors, try pointing a light at the ceiling to create soft light.

Have a great trip!

Update: The sample shot you added is a great example of where keeping light in mind could help. I'm sure that long set of lighted arches was lovely, but it's much brighter than anything else in the photo, so it tends to make everything look dark and dull. Composition-wise, you were back far enough that you got a lot more dark sky and hedge than you really need, and it seems a bit unfortunate that the subject's head is right at the vanishing point. If you could do it again, you might move your subject a bit closer to the lighted arches (or even under the arches), and get yourself a lot closer to the subject so that you could use that soft glow to light up her face and eyes. Even with the subject standing in the same place, just taking a few steps closer would fill the frame with more of her and less of the sky, and it'd get her head away from the vanishing point formed by the arches and hedge.