Apart from being aerobically fit there is not too much you can do to prepare until you get there.
Most people don't develop any serious symptoms below about 8000 feet. You may notice a slight shortage of breath when exerting yourself. If your plans don't take you above that I wouldn't worry about it too much. Commercial airliner cabins are kept at the equivalent of 5-8000ft.
If you are going about 8000ft for very long then first thing is to understand altitude sickness.
Plan on acclimatizing for one or two days once you get there before you attempt any major exertions and have an alternate plan if it turns out you are particular sensitive. You can get a prescription medication, diamoxx, which can help if you know you are sensitive or as a precaution. Otherwise ibuprofen has been shown to be helpful assuming your health allows you to take it.
It is very important so stay hydrated because it is much drier in the mountains. Also sun protection is very important. The altitude exposes you to much higher levels of UV.