I have an old mountain bike that i would like to start riding again. I rode it today but there is something wrong. It will not shift out of the highest gear. When i move the shifter it makes some ticking noised but does not shift. As you can imagine its a pain in the ass riding in high gear everywhere I think it has something to do with the rusty cable lol but I truly know nothing about bikes. I would really like to fix this myself and not take it to a shop. Can anyone help?