The knight is the obvious place to start, because I think the reason many men choose the medieval theme or agree to it, is that they are drawn by the strength and valor the the medieval man represent. The knight is the ultimate medieval warrior and real ladies man (plus you get to carry a sword!).
The title of knight could be bestowed anyone by the ruling king or queen, but by late medieval times the title was considered preserved for the nobility and those born into wealth. The thriving litterature ascribed the Knight a symbolism that idealised the title bearers. They became the perfect gentlemen, christians and guardians of justice. Today women sometimes say that they love when a man acts chivalrious, which is an ideal cultivated in the Middle Ages.
Now for an eksample of an outfit: I love this combination put together by Etsy and Ebay-seller Morgana's Collection. It's not as over the top as it could be but the elements: chainmail, helmet, tabard, sword, leather belt and shoes and gauntlets make an accurate and cool impression.
The tabard is like an overcoat that you wear over the chain mail to show off your colors. The colors could depend on the country you fought for, (like England was red during the crusades and the french was blue), they can represent a particular order of knights like the Templar Knights (white tabard with a red cross) or the order of knights of st john (black with white cross). Yet another background for the choice of colors could be the family crest, which often had a particular set of colors or animals that was embroided on the tabards and capes.
A tabard is fairly easy to make - a big rectangular piece of fabric with a hole for the head - or fairly cheap to buy. What I like about Morgana's Collection, is that they can make you a tabard with any color or pattern you can think of.
Gauntlets, shield and helmet are not necessary, but like with women and their jewelry - the more details - the better. A belt and leather shoes are however essential to achive the look and sword will definatly be the frosting on the cake. :)