Assuming that the food itself is kosher, you would have to serve the food on kosher dishes using kosher utensils. If you don't have a kosher kitchen, using plastic and paper is usually fine. To be safe, cover the table the food is sitting on with a sheet of paper or plastic.
If you're asking how to prepare kosher food, that is more complicated. You would have to have access to a kosher kitchen as any contact with non-kosher utensils (this includes ovens, microwaves, counters, tables, etc), would render kosher ingredients non-kosher.
The basic rules to follow are:
- Any meat or poultry has to be certified kosher.
- Fish has to be from a kosher species, skin on. If the packaging the fish is in has kosher certification, it doesn't have to have the skin.
- Meat and dairy cannot be combined in any way, poultry qualifies as meat in this case. Even products that contain dairy byproducts (whey, etc) cannot be combined with meat.
- Fish and meat cannot be served on the same dish or at the same time.
- In North America, milk is considered acceptable by most groups. Some groups do require kosher certification for milk. Almost all require kosher certification for dairy products (cheese, etc). There is a higher level of certification for dairy products called Chalav Yisroel.
- Any processed ingredient must have kosher certification.
- Some groups do not eat certain vegetables because it's too difficult to clean them of all bugs and dirt (broccoli, cauliflower, romaine lettuce, asparagus, etc).
- Some groups require that a Jew is involved in the food preparation process of cooked food in some way, even turning on the stove/oven qualifies.
Depending on what's being served and where, some people require supervision by a mashgiach. A mashgiach is a religiously observant Jew who is well versed in the laws of kashrut.
