The nice thing about John Donne is that he often picks a single idea and just runs with it. He beats it to death and then beats it some more. In this case, that single idea (or "metaphysical conceit," if you want to get technical) is that the mixing of blood in the flea is somehow similar to the consecration of a marriage through sex. If you keep this idea in mind while reading, you'll make it through the poem just fine. That said, you might want to have a dictionary handy to look up words like "maidenhead," "cloistered," "use" (it's not the standard usage of this word), and "jet" (black).