Would sum1 be able to help-problems making a thumbnail

I know this sounds daft but I must be really bad with computers! I cant seem to make a thumbnail, everytime i upload a thumbnail it shows up really blurry. No one else seems to be having this problem. I tried going to image size and setting the resolution pixels to 72 then went to height and pressed 50 and the bottom one changed. I then saved it as save to web and this screen came up, frm then on I didnt know what to do? Could anyone help? :-/