Dont worry about not knowing something, we all start that way and either bungle are way through or get advice
(someone will probably write a reply to this telling me im mad but each painter has there own methods most of the time)
By priming a model you basicly give the model a flat coat of paint as a base to work off, different painters prefer different colour undercoats though most use a grey primer and build up the colour from that
if you dont have an air brush the easiest and cleanist way to prime is to buy a spray can of undercoat you can get these from games workshop (black or white undercoat) or I would recomend from a company online called 'The Army Painter' they do various colours for undercoats although I would always recomend sticking with either grey, lack or white as your undercoat as there other colour undercoats dont seem to take paint over them as well.
I cant stress enough how easy it is when using spray can undercoats to over do it and fill in detail, so be carefull and remember less is more, several VERY light coats is better then one thick coat!
If the model your painting if mostly one colour (red for example) and you dont have an airbrush then a lightly applied white undercoat, then using a red spray to go over it will save you the hassle of using a brush to paint the red in and the spray leaves a smooth clean finish without any brush marks
Hope this is helpful, good luck