post's format

Whatever the informations I give like aligmnent etc… It doesn’t work.

please post an exact example.
where are you adding the css?
what are you trying to align?

could be that the stylesheet is broken by an invalid css code;
general, there are two major validation errors in your linked site:
http://validator.w3.org/check?uri=schoolsofhopefoundation.org%2Fblog&charset=%28detect+automatically%29&doctype=Inline&group=0

http://codex.ww.wp.xz.cn/Validating_a_Website

with the list of the posts where the heading of each goes on the left if the one before contains an image.

for instance, try to add this to style.css:
.hentry:after { content:'.'; display: block; clear: both; height: 0; visibility: hidden; }

(not widely tested)