I want to create a new network dataset from a street layer. Should I create it in a normal file or in a personal geodatabase?
A network dataset must reside within a feature dataset. A feature dataset can be created in either a file geodatabase (folder.gdb) or a personal geodatabase (filename.mdb). You can't create one in a shapefile, if that's what you mean by 'normal'. Note you'll need to add the street layer to the network dataset in order to build a network from it, and there may be some cleanup necessary/involved in building a useable network. You may want to run through this ArcGIS help tutorial.