Glovis is one of the best places to start compiling free satellite imagery. For a new user, LANDSAT imagery is a great place to start - you will be able to find data covering the 1970's to present day. There is also a wealth of information available for working with this data. For example, if you are using ArcGIS you can quickly learn how to develop a landcover map. This page lists some free data viewers that you may find useful.
Here is the Quick Start guide to using GLOVIS.
Here is some supplementary GIS data specifically for Cambodia.
Even more Cambodia data in the form of an interactive geographic database. This one seems to focus on historical information, but does include free satellite imagery.
I saw a blog post from developmentseed for their command line utility landsat-util.
Power tools for Satellite Imagery
The landsat-util can be forked from github and compiled from source unless your OS offers it in a binary ready to go.
The blog describes it simply as:
a command line utility that makes it easy to search, download, and
process Landsat imagery.
You can search based on date, cloud coverage % and other things, download immediately, or process once it's downloaded like pansharpen or stitch the images together.
You can preview images before you download. Search commands provide a
link to a thumbnail for each image.
landsat search --cloud 4 --start "August 1 2013" --end "August 25 2014" country 'Vatican'
Using the --pansharpen flag will take longer
to process but will produce clearer images.
landsat search --download --imageprocess --pansharpen --cloud 4 --start "august 11 2013" --end "august 13 2013" pr 191 031
You can also perform all processing on images that you previously
downloaded.
landsat download LC81050682014217LGN00
landsat process --pansharpen /your/path/LC81050682014217LGN00.tar.bz
Here's a readme with more info.
Best Answer
For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).
Primary download site for Sentinel
https://scihub.copernicus.eu/dhus/#/home
alternative solutions can be found on Where / How to download Sentinel 2 Images
If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.
PROBA-V : http://aida.vgt.vito.be/content/products
MODIS (and also Landsat, but for Landsat only you could use Libra as mentioned by @Richard Law): https://earthexplorer.usgs.gov/