Before you walk on the catwalk I am afraid you have to go through castings and indeed get represented by a model agency. In Europe you have to be over 18 to be able to walk the runways, and you have to have the height and build as well as the looks - all of which I am quite sure you do. Best place to start is to go to a modelling agency - Storm, Select, Models One just some of the big names with a few photos as well as yourself and ask them if they would consider taking you on, what you need to do etc. Do NOT pay for a portfolio to be taken, at the early stages non professional shots can be enough. If you get taken on then the castings start. This is when designers ask you to walk for them. Erin O'Connor syas that she imagines she has wing mirrors on each hip when she is walking - this is a good way of describing it. Watch her, Naomi, Gisele, the greats in action on style.com and shoulders back, head high and off you glide. Good Luck!