Corrections and suggestions.
(If you haven't started yet then present progressive (have decided) is best. If you have started (or even finished) then simple past (decided) is best.)
During my IUTís studies, at the end of the second year, I had the opportunity to do a training period in enterprise for ten weeks. It was for me a good experience of the real condition of working in enterprise; besides I have learned many things about the development of software and in working as part of a team. I have never do part time jobs but during the four last summers I have work with a friend in a conception of a web site. Itís a real good work experience for me. I never do part time jobs, but during the past four summers I have been working with a friend developing a web site. Itís been a really good work experience for me.
Sorry for the late reply.