To get it started, give every house on the block a flyer - make it colorful and fun and make sure it says BLOCK PARTY in big bold letters so that they're sure to read it before they throw it out....you may even want to include your street name, i.e., ELM ST. BLOCK PARTY! Then give a brief statement of what you're trying to do - " Come join your neighbors for some Halloween fun!" Then list some of your ideas as bullet points - suggest a date and give your phone number and email. Be sure to solicit help in forming a committee (or several).
A block party is a great way for all of your neighbors to get to know each other - especially since technically, no one is "hosting". They may feel more comfortable meeting in the "middle" i.e., out on the street. Plus, they have the added reassurance that they can escape home at any time! Each family should be responsible for something...a craft, game, etc. - this not only spreads the cost/tasks, but it also forces people to mingle from house to house. Take a poll as to how many would like to make the food Pot Luck/Buffet style vs. how many would rather just provide their own meal/snacks. You could also do it on the style of a Progressive Dinner...snacks at one house, beverages another, main meal (chili or burgers - something easy), desserts. Encourage everyone to invite friends also (again, this will make everyone more comfortable) Kids are great ice breakers themselves, so enlist their help...they'll drag the adults around exploring. Encourage everyone to decorate and to wear costumes -it's a party after all! Don't forget music! I guarantee everyone will end up dancing!
Keep in mind, if you intend to actually close the street, you will need a permit from the local police station and there may be a small fee. If not, have it on the front lawns, or sidewalks as the case may be.
HTH!