You may want to check out the outdoor theater forum. Otherwise you need to provide us with a lot more information like your budget, what size screen you want to project onto, etc. the folks here are a very nice bunch and very helpful. Just need a bit more info to help. Best place to start is the outdoor theater forum. You can get some ideas and other basics there. Just a ballpark... A new projector can start as low as $400 for a decent one on up. If you want cheaper you'd have to go used which has its risks. Also you'll need to hear the movie so you will need some sort of stereo/speaker setup.... Not to mention a blu-ray or DVD player.
I actually watched a movie outside just a couple nights ago. used my laptop and an old projector and I have to say it would probably be a huge waste of money for me to do anything more than that. maybe if I lived out in the country, but there's no way I could enjoy a movie the way I want to see it without pissing off a few neighbours, haha. but then again, it's doesn't get dark enough to even think about watching one until about 10pm here. or else it's winter...
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One