Preparing a cabinet for painting is quite basic, but could involve a bit of work.
1) Cleanliness – A clean surface free of grease and grime is necessary. Use a strong cleaner like TSP (trisodium phosphate). This is a strong cleaner that will remove any deposits. Make sure to rinse well with plenty of fresh warm water.
2) Sanding – This is a very important step. Usually, hand sanding with medium/fine sanding sponges is enough. The idea is to dull the sheen of your existing finish. Creating deep scratches with a rough grit isn't necessary. Make sure to remove all the sanding dust with a vacuum and tack cloths.
3) Prime – Use a good fast drying oil base primer. Applying two light coats is a good idea. Allow to dry overnight before proceeding.
4) Lightly Sand Again – It is a good idea to lightly sand again to smooth out the primer and reduce the brush marks. Remove the sanding dust as before.
After doing all of the above you can either caulk the cabinets to the adjoining wall or start painting. It depends on your cabinets and the type of trim used against the wall.
I like to use oil base enamel for the finish. It provides a nice smooth finish and it harder than most acrylics. Oil based paint will take much longer to dry when compared to a good acrylic. If time is a concern use a very good quality 100% acrylic paint instead.
Use the following articles and answers for more information;
→ <a href="//questions.house-painting-info.com/painting-a-bathroom-wood-vanity.html">Painting a Bathroom Vanity</a>
→ <a href="//questions.house-painting-info.com/what-kind-of-paint-should-i-use-for-bathroom-cabinets.html">What kind of paint should I use for bathroom cabinets?</a>
→ <a href="https://www.house-painting-info.com/refinishing-kitchen-cabinets.html">Refinishing Kitchen Cabinets</a>
→ <a href="https://www.house-painting-info.com/article-simon_phillips-paint_bathroom_vanity.html">How To Paint A Bathroom Vanity Cabinet</a>