I don't want to go in winter time. When will be the best time to go? April, august or september? I want to plan to stay a month but I have no idea about costs. I am looking for some place affordable
Europe is a pretty big place (between Sweden and Greece, Ireland and Bulgaria) with as many languages as countries (a few sponsor more than one). Your foreign languages abilities should be considered in your choice. Whereas in most mayor cities you may get along with English, not speaking the local language usually limits results when negotiating for information. Travel and living costs until mid-May and from mid-September onwards are more reasonable than during the remaining periods. April can be pretty chilly up North, August is the main holiday season in most countries, whereas September usually has better weather South of the Alps.
Above factors will influence your expenditure and should guide your choice of times and places.