Dale Dougherty of Make magazine gave some simple ideas for starting your makerspace at ISTE 2014's EdTek Tips. To summarize them even more succinctly:
1) Get space
2) Get stuff
3) Get out of the way
Finding space can be as simple as freeing up a rolling cart or book shelf, or allowing students to use workrooms or office space. Shannon Miller mentions giving up her little used office, I set up messier projects in my workroom.
A makerspace doesn't have to start with 3D printers and circuit boards. Dougherty suggests just putting a pile of stuff out and telling kids to create.
And finally, don't overthink the outcomes. Kids are supposed to be tinkering, not coming to you for instruction. Make it collaborative by having them work with others if they're stumped.
Remember, a makerspace doesn't have to be curriculum-driven, and there are as yet no standardized tests. Spread that message to students and get them in the door to imagine and create!