Well I know what I've done in the past, but first and foremost: have patience. Sanding and prep work are everything when you paint. Yes I am quite aware I just uttered one of the most used cliches in automotive history but it is so very very true.

First things first: wash your car. You probably knew that but I can never be too sure.
Next be sure to remove any wax on the paint. I used mineral spirits but I know there are products out there.
Is your car clean and wax free now? Good. Now check again and ask yourself 'Was I half assing it?' If you didn't then resume by removing ALL your trim pieces. Yes. All of them. Lights, side markers, side mouldings, etc. Don't slack and try to sand/paint around those pieces. You will fail. Now ask again 'Am I half assing this?' The reason I have to ask is because I have half-assed a paint job and ended up doing everything over again. Could have saved myself a lot of time and money if I had done things right the first time.
Now the fun part. I hope you're patient. You will be hating life for the next few hours/days.
Next tape off anything you don't want to sand. Scuffing up badges and the like is not good times.
Now you have to decide. Are you going to paint on top of your current color? If so grab some 320 grit sand paper. If you're going down to the metal, grab some 80 grit and prepare to cringe as it eats into your paint. I've never sanded a car down to the metal so I can't really offer any input on that subject so you're on your own if that's the route you choose.
Get your 320 grit and a bucket of water. Why wet sanding? Wet sanding will give you a much smoother finish and be easier to paint on since you don't have to worry about having left particles on the paint. Also it keeps you sanding because you think you're done after 2 hours since the paint looks dull. You're not done. You're just looking at removed paint. I used a block sander on the larger flatter surfaces and hand sanded any small contours and curved by hand. Why did I hand sand? I don't own a power sander because I'm poor. Sand and sand then let the car dry or dry it yourself with microfiber. Does the paint finish look dull? Kinda? Keep going. There shouldn't be any shine anywhere. Not even a speck. You'll know what I mean when you start. It will take time so be patient. It will pay off. After a day or so of nothing but sanding your arms should be sore but the car should be dull as a result. Be sure that once your surface is dull you don't continue sanding. You just need to make sure that the primer has a surface to bite into. Now get your air compressor or borrow one and use it to blow any dirt and crap out of spaces in the car.
Look one last time to ensure that your taped off parts are still taped off. You will probably need to retape those areas since the water from the wet sanding has probably taken the tape off. Make absolutely sure that you've taped off everything and there's no leaks. Painting something you didn't intend on painting will make you sad. I know I was.

Now the car's ready for paint. At least it should be anyway.

Total cost: sandpaper + sore muscles + tape = paint job.

Also try just researching around online. There's lots of information on how to DIY paint a car. That's what I did. The big things I learned. Be patient, be fearless, and be happy. Be patient because you will be working on a mind numbing job for hours on end. Be fearless because if you're constantly afraid of screwing something up, you will miss parts that needed proper prep work. Be happy because if you get frustrated you will not being concentrating on the task at hand. Just think how pretty your car will look soon and persevere.