1. Appearance - personal Get a Haircut - Unless you are an Italian model or a porn star,
Long hair is out. On most guys its unattractive, almost always greasy, and usually badly styled. Very few girls prefer long hair on guys (I'm sure many can tolerate it, but why risk it). You might be the exception to this rule, but strongly consider whether you in fact are - Many guys go through a long hair phase around age 17-19, very VERY few people in their mid 20s look back and think it looked good.
2. Appearance - clothingGeneral - Generally, as a guy you should have some slightly smarter clothes for going on dates etc. An outfit that you know looks good also does masses for your confidence.
Tops - First of all, black t-shirts or 'comedy' t-shirts (and even worse, t-shirts with dragons, fantasy scenes or wolves on them!) are horrible and mark you out as a basement-dweller. AVOID. Next, while hoodies are awesome to keep warm while doing exercise, or when slumming it after a massive night out, they aren't a top you should be wearing out socially (you wouldnt wear tracksuit bottoms out would you? Its the same concept). What you wear on your top half does depend on your personal style, but normal people will wear either a t-shirt with some kind of design on it, a polo shirt or a real shirt (NEVER SHORT SLEEVED SHIRT!). I personally advocate the last but its a matter of personal taste and how smart you need to be. For warmth go with a jumper, jacket or coat. Never wear waterproofs unless you are going hiking - people in North-face all-weather hiking jackets in the city look ridiclous. Use plain t-shirts underneath to layer up if needed for warmth.
Glasses, Hats, watches, jewelry etc - This is a matter of personal style of course, but most people in glasses would look better with contacts. If you think they are too expensive, have a look somewhere like
www.daysoft.com (very VERY cheap daily lenses, you'll need to get an eye test at boots first to find out your prescription though). Most guys can't pull off jewellery and just look stupid, especially wearing chains, rings or earings or other face piercings, matter of taste though.
3. AttitudeIf you've got sections one and two wrapped up, likely you are feeling a lot more confident about yourself. This is key. The single biggest factor in being attractive is being confident. Even if you aren't yet confident, you will get more confident in time through practice approaching girls and going on dates.
Whenever you hear a guy saying "why dont girls like 'nice guys'" you can be 100% certain his key problem is one of three things - that he isn't, in fact, a nice guy, that he is not being forward enough in his approach to girls or else he is too keen.
Points to remember:
Don't be too Keen - Desperate guys are a massive turn off. Coming on too strong likewise - both imply that finding a girl who likes your attention is rare so you are making a big deal out of it, this is NOT a good impression to give. To combine this with the point above, the attitude you are going for is "I want you, but If you aren't interested I'll go after another girl instead".