The three most essential elements of any barbecue party are the guests, the weather and the food. I can not make suggestions about your friends and family, you are on your own there, but I hope you will find my suggestions for the other two fronts practical.
An impromptu party in the garden is great and often the best parties are the ones that just happen off the cuff, but if you want something a little more formal or more impressive then you have to make plans. The first move is to pick a date far enough in the future for most people to be unlikely to have a prior engagement, but not so far that they might forget about your barbecue party.
Depending on where you live of course, try to pick a date when it is unlikely to be cool or wet. This is hard in places like the UK, but may be easier where you live. If the party goes on into the night it may get cool anyway and you could hire a few gas patio heaters in advance just in case. You could also have a sunshade or sheltered seating area in case of blazing sunshine or a light shower.
These days a lot of people are vegetarian and you will have to prepare in advance for them and other dieters, if they make up a sizable number of your guests. You could send out RSVP invitations and ask your guests to make known any particular dietary requirements. I am not really suggesting that you have to cater to everybody, but if you have a lot of Jews or Arabs coming you will need to offer an substitute for pork chops and of course there are other minorities too.
In fact, you may have to do some research on the religions of the people coming to your barbecue party, because often cooking utensils may not have touched pork of meat or shellfish. Most people of the guests with these dietary needs will have been in this situation before and will understand if you go some way to offering an alternative.
For this reason, you will have to get your RSVP cards back at least a week before your barbecue party and do a bit of investigation on the telephone or the Internet. Then start preparing the barbecue side dishes. These should include baked or and boiled jacket potatoes and potato salad, baked beans, egg quarters and coleslaw along with pickles, relish, onions, chutneys tomato and curry sauces (hot and not so hot), as most people will eat them. These can be prepared a few days in advance and kept in Tupperware containers in the fridge. Bread in the form of buns is also important so that people can make sandwiches.
It is imperative for a slick operation, to have your party area fully prepared before your guests turn up. it is a good idea to set the music system up in the garden shed or the garage to protect it it from any sudden shower. Provide plenty of tissues for wiping greasy fingers and plenty of receptacles for the debris. Finger bowls with lemon water would be useful for this objective too.