Hi Olaniyi Oluwatobi Emmanuel,
Sorry to take so long to respond to your question. You should be able to get free access to Landsat data from this link here:
http://glcfapp.glcf.umd.edu:8080/esdi/index.jsp. I've only ever used the marine counterpart, so I've no idea how easy this data portal is to use.
In terms of specific instuctions on using the images, can you explain a little more about what exactly you would like to use them for. For example, are you wanting to use them to define the boundaries of habitat patches or do you want to do something more complex? Also, what part of the world are you working in?
If you can give me this extra information, I can see if I can help. However, as I said before, I've only ever used the marine data before which may limit how much information to provide.
All the best,
Colin
PS You may find at the moment that access to satellite data is restricted due to the current close down of the US government due to budget problems.