you are a classic ectomorph. Tall with a slight build. You can gain weight, but forget about gaining fat by consuming it. you want to build muscle. What you need to do is force yourself to consume more calories than your body needs. This is difficuld to do, i know because i too share your problem, well to an extent. I'm reading this information from the Bodybuilders Encyclopedia written by none other than Arnold Schwartznegger (Mr. Universe many a time).
Protein and calories are what you need. Get off the junk food, seriously. You are just like me i bet. eat anything and don't gain a kilo, right? envy of your frineds! so, you need to take a bodybuilders suppliment in the form of a shake (from your local health store) and follow the directions. Take it in between your daily meals. this is very important.
buy some weights and do some training. all these extra calories need to go some where, so why not in muscle development? this will assist your riding also.
do as little physical activity as possible. by this, i mean try to avoid highly active sports like basketball (sorry), because you are just burining up calories that are better suited to muscle development. you can get away with it, but you must have more than enough protein/calories coming in.
So like i said in the start, get off the junk food as this wont help you at all. no fried foods or big-macs. health food buddy, health! looking at Arnies book, he recommends a pretty heafty diet for the Ectomorph, and i will type it out if you are interested, because it's pretty long. But basically, he recommends massive amounts of milk (yuk!), meat (or fish, fowl, cheese), salads, vegetables, and a bit of fruit. This, along with the body builders suppliment, and a bit of weight training, should get you there.
Half way through last year, i was about the 63kilo mark, and i too am 180cm. Now, i weigh in at 70kg's, and even though this is in our healthy weight range (66 to 78Kg from memory), i still want to gain more.
So let us know how serious you are, and i will elaborate as much as possible from Arnies book.
Good question though. eveyone seems to concentrate on how to lose weight. most people don't seem to realise that some poeple struggle to gain weight.