just had walls skimmed when and how should i paint

DO NOT PVA YOUR WALLS .. who came up with this idea ? You need to make sure walls are not dusty so a wipe down with a damp cloth is a painful but preventative measure. Francis is spot on - do not use a vinyl emulsion, do a mist coat, or sacrificial coat as my dad used to say, of emulsion and about 25-40% water. As plaster is very absorbant you will need to work in the paint - that is roll over the area a few time to spread it out and work it in. If plaster has been over-polished you will need to lightly sand it otherwise emulsion has nothing to key to. Over-polished areas will stand out after the mist coat - the paint will not have adhered. After mist coat apply 2 top coats (not diluted, straight from can). Top coats can be vinyl emulsion but mist coat should be bog-standard emulsion - like Crown trade emulsion from B&Q, about £16 for 10L.

Apply 1 watered down contract emulsion (not vinyl) 75% water 25% paint, bit messy so make sure you have sheeted up 1st, then follow up with 2 full coats of contract emulsion. *contract emulsion lets the plaster breath, where as vinyl forms a layer of paint on the surface..

what ever you do ,dont put pva on the walls ,or your paint will end up coming off i tend to 50-50 water and paint still a bit messy, but make sure walls are dry even with breathable paint its never a good idea to put paint on wet walls you end up with the odd water stain