We had a big one last year which was very successful.
We put a marquee up that we bought for about ninety quid from amazon, but it's huge, you can get smaller for less money, much less money. You need something in case it rains. Simply you never know.
Lots of drinks out and ice buckets. We used decorator tables that we borrowed. Everyone who brought drinks we put them on thr tables sitting beside what we supplied and let folks help themselves.
Plastic cups for drinks and plastic cutlery and disposable bamboo plates ( they are solid and hold their shape). Then you just walk about after they eat with a bin bag and clear up so people can keep drinking.
Music, make a playlist and have a speaker outside, use an extension cable If you have to.
Beg, borrow, steal tables and chairs. People like to sit down and not on the grass ideally.
Buy some big spotlights if your garden isn't lit up, just plug into the extension with the speaker. They are dirt cheap from amazon or b&q.
Plenty of salad, rolls, coleslaw, potato salad , as well as things like mayo etc. Cook things like chicken, kebabs, sausages etc in the oven and put in those disposable trays you put turkey in to cook for Xmas. Do things like burgers on the bbq itself. If you have a big bbq also do things like mushrooms and other veg etc on there. Make sure all the food is ready at once and bring it out together.
If you think about it, and we have a big eight burner bbq, you can only cook so much on it at once. So for example, forty people, you need thirty burgers, thirty chicken breasts, thirty kebabs, thirty sausages etc min, it would take forever to cook them all on thr bbq in rotation, so some things need to be in the oven. Have everything marinaded and on trays before it all kicks off so you just need to put things in the over or take them out to thr bbq.
Don't scrimp on portions or drinks. Make sure you've a ton of mixers. Make some sangria, some Pimms in big jugs. Plenty of beer, we filled large plastic storage boxes with ice and put the beer in that. Lots of wine, red and white, some bubbles if you're feeling extravagant. And as said, a shit load of soft drinks.
Also we put loads of nibbles out. Olives, nuts etc on the tables, for people to pick at before we ate.