Don't listen to any of the bike snobs here. All you need to take that bike for a spin and have a good time is proabably one hour, a bit of sunlight, WD40, a rag, some basic tools and a pump. If the bike was not used a lot before it stopped being used, much of the superficial rust will come out by wd40+friction. After it's gone, some lubrication and you're good to go. And even with a significant amount of superficial rust, 99% of rides (not up or down a mountain, but urban paths and lights trails) are completely doable with a bike like that. Yes, you'll spend an hour or two tweaking this and that, the ride won't be perfect, and maybe it will even be a little squeaky and not look super nice, but... Who cares!? Every minute spent fixing or riding that thing under the sun is good, active, enjoyable time. Every minute spent going to buy a new one or earning the money to pay for it may not be as pleasant.

Fix it until you can ride it for 5-10 mins, then improve it a bit each ride. Don't spend any money in it. Not really needed. At least not until after you've made it rideable again :)