Decide, by what right are you going to prove their nationality. There are two options: a law of blood and law of soil. In the first case, it is sufficient to collect the documents that confirm your kinship with the representatives of the nation need you, even if geographically these representatives do not live on the territory of the national state (e.g. Jewish by birth, living in Russia, the legislation of Israel and still be considered Jews). On the contrary, if your ancestors for a long time lived on the territory of European States (e.g. Poland), you can prove this fact his attitude to the nation of the poles.
Take the birth certificate of their parents or grandparents. The nationalityspecified in them, will be yours, adjusted for the particularities of some countries. So, if in Russia belonging to a nation is determined by the father in Israel - mother. Attach a birth certificate of the father or mother documents proving your relationship, and with this set, please contact that organization, which is required to provide evidence of your nationality.
If you need to prove the nationality for expatriates and documents confirming your right to receive citizenship in another country, not preserved or nationality they are not specified (documents of pre-revolutionary), see the specialists. Make a request to the Consulate or Embassy of the country where your ancestors lived. Along with a petition for repatriation request information that in a certain period of time the person who is your direct ancestor lived in the territory of that state.
Ask for advice in the community or at national-cultural autonomy of the nation, belonging to which you need to prove. Either hire a lawyer specializing in the repatriation.