Always prime your models in a well ventilated area. Wearing a little face mask is a good idea as well, especially if you're going to do a lot of priming and painting!
Acrylic paints don't 'stick' to bare metal or plastic models very well. You will end up with a patchy, blotchy paint job and it will be difficult to apply and control the paint on the model in the first place. Primers have slightly different formulas to their paint equivalents. They are better designed to adhere to the model (or surface) they are sprayed on, and form a better bind with paints applied to them. Additionally primers can help smooth the surface further.
There are a number of different primers you can use, not just ones labelled for models. The most popular ones I've seen have been;
Duplicolour Sandable Primer
There are lots of primers out there so don't feel limited to just using something from Games Workshop or the like, because they can be unecessarily expensive. But if you don't end up branching out, do a quick google to see if other people have tried it and how they got on with it so you don't end up wasting money unecessarily.
Primer needs to be matt - this is the main thing to look for when trying random sprays!
The easiest colour to start with is black for most people, but you can get primers in any colour and finish. One of the great selling points about Army Painted primers, besides being half the price of Games Workshop's, is that they are available in a huge range of colours. As are auto-primers, like dupli-colour and halfords own brand.
No matter how you end up spraying your models the first few steps are going to be the same; make sure the weather and humidity is suitable, shake the hell out of your can, put your minis on something you don't mind getting covered in paint, and do the spraying somewhere well ventilated that you can also cover in paint.
Primers are labeled with conditions they work best in - outside of these you're likely to end up with patchy or chalky lumpy spray. At the very least never spray in the rain, the extreme heat, or much colder than room temperature. You should shake your can for at least a minute, more if it's new or hasn't been used in a while. This mixes up all the substance that has settled in the bottom of the can. Cardboard and newspaper are good for spraying on.
Now for the actual spraying!
The Ideal Way!
Keep your can of primer upright and spray parallel to your models. This means having your models upright too, or spraying paint straight up their butts.
Either spray them in there based, or stuck to masking tape. Double sided masking tape is the easiest for that, or you can use single sided tape stuck sticky side up by more sticky tape!
This is a neat trick to use sometimes regardlss of how you're spraying because some model parts can be so fine and light that they get blown away by the power of the spray.
Prop this against something and spray away!
The Less Ideal Way!
Spray them however the hell you want. Keeping the can parallel to your models will help keep the spray even across them. It's easier to spray with the can away from vertical the more full it is, as you don't have to worry about where the straw is in the paint.
Even if you're spraying your models whilst hung upside by your ankles like a bat make sure you always keep these in mind;
PHOTO of upside bat spraying
Don't spray to close - 6 to 12 inches to start and change as gives you the best results.
Don't spray in one huge, continuous splurge of paint. Use of quick sprays instead. Helps you save paint as well as not drench everything.
Kept the mist fine - don't spray a model so much it starts pooling on it. Unless you're going to have a little nameplate on your Dreadnought saying 'Mr Lumpy'. Do a couple of fine passes over the model - instead of that huge splurge!
PHOTOS OF GOOD AND BAD SPRAYING.
The sound the can makes when you spray should be uniform as well - any popping or spluttering means either you're not holding it at the right angle for how full it is or its not mixed properly and your not getting the right ratio or paint and propellant out.
Try shaking it or changing position. If one doesn't work try the other.
Try and keep your primer layer fine. The thicker it is the less detail on your final model.
Leave the models until they are completely dry, then turn them over. If you've sprayed them in their based you can just spray the whole model in one go.
Sometime you'll only notice mold lines and flash once the models have been primed. Don't worry about this - just remove them as usual and if possible give them a very fine spray to cover up the clear spot. Or go over with a couple of layers of thinned paint.
And don't worry! If you mess up your priming job it is removable from both plastic and metal models. Try, and if it goes wrong you can try again :)