Too many blank lines under header

Please, can anyone help me? I think word press is very difficult, but I’ve made it pretty far since yesterday.
When I added widgets to my left sidebar, it made blank lines below the header, so there is a large blank space before my blog begins. When I go to the edit template I can’s see anything where I can delete some lines? I know a little about dreamweaver, but I’m thinking it may be the theme page I chose.
thanks.