I find there is an idea heaps of young guys get and that is to send my mixtape out to clubs. This would very rarely work, you really need to focus on one or two clubs you want to play at, get to know the other djs there and the promoters introduce yourself to them let them know you are keen. and then like Mitch said scope out the style, tempo and types of artists djs play in the early slots, then go make a suitable mix for an open slot there.