As The Builder mentions above, it depends on what the finish is at the moment. If it's varnished wood, then lightly sand it down and wipe down with a damp cloth. If it's bare wood, then you'd need to give it a coat of primer or undercoat.
Apply a coat of black gloss paint (either high gloss or satinwood, depending on the finish you want). To achieve a really good smooth finish, I'd recommend you use a little paint roller (say the 4" size, smooth finish) - that way you won't get any brush marks, but you may need to put on several coats so it might be worth trying to find some water based gloss paint (B&Q do these in their own brand range). If the trunk is likely to get a lot of bashes, then you could give it a top coat of polyurethane clear varnish to protect the paint.
International do a lovely Japlac High Gloss Enamel Paint that comes in small quantities (125 & 250ml) that might be more suitable for your trunk.
Another way of painting the trunk would be to use one of those aerosol sprays - you might need several sprays to achieve a good finish (you also might want to do this outside, on a dry day).