I just painted all of my kitchen cabinets a few months ago. They were wood that was already painted, but it's still the same even if they have never been painted:
I used a 2in wide slanted paint brush on the narrower areas and a 4in. wide slanted paint brush on every other area. It's worth it to invest in a quality brush. There are plenty of DIY guides on how to properly paint with a brush and how to wash them properly.
I bought Sherwin Williams paint during their 30% and 40% off sales which they have pretty much every other week. If you join their free Preferred Customer group on their website you get an extra 10% off. They also have $10 off of $50 coupons on their website that you can print out and use in conjunction with the 30% off sale. I got the ProClassic Interior Acrylic Latex Enamel, Semi-Gloss (I don't like paint to reflect light as much as Gloss and High Gloss do) for the cabinets. It's thicker than normal paint so it's a little more work to get it painted on, but it settles and doesn't leave brush lines after it dries.
1. Remove all the cabinet hinges and door handles and lay all the doors on top of painter's canvas in a less used area of the house. Obviously it's going to take longer to get the doors done than the actual cabinets since you have to wait for them to dry before you can flip them.
2. Clean the cabinets & doors with water or soap and water.
3. Sand the cabinets & doors (this is so the paint clings better to the surface).
4. Clean the cabinets & doors again with water or soap and water.
5 Use painter's tape around the edges of the cabinets.
6. Paint a white primer on the cabinets & doors (optional, but I like being thorough)
7. Wait 4 hours minimum then put on your first layer of paint.
8. Wait 4 hours minimum then put on your second layer of paint (also potentially optional, but I think it makes the paint job last longer).
9. Carefully remove the tape from the edges of the walls.
10. Re-install your old hardware or install new hardware on your cabinets (I changed out all the handles and opted for semi-closing hinges over our previous loose hinges)
I don't know if I forgot any steps. I think that's about it.