first you are going to want to clean up the fence before you paint it. Get a paint scraper or a wire brush and take off as much old paint as you can, especially the chips that are coming off. If the fence has mildew you can wash it with a diluted solution of tsp or bleach to water (not sure the ratio, but prob a couple capfulls to one gallon H20. After it is cleaned and dry you can paint it with an exterior paint, you can use latex based (water based) or an oil based (which makes cleanup a little harder, but generally goes on thicker.) I would just get an exterior latex. Any sunny day that is above the 50s or 60s should be fine, it will dry faster in the summer but it really doesn't matter. If the fence has wide enough flat areas you can buy a roller to save you some brush-work. just roll everything and go back with a brush and touch up. Hope this helps.