You should try and design in black only at first - a logo will be used on all manner of things from stationery, adverts, T shirts, web, mugs, towels etc. Try sketching (pencil and paper) it will help you get a feel for the designs better. Know who your target audience is - male, female, fitness or body building. Look at what other gyms have as logos. . . As Sparks says just the name/dumbell to start with. . . KISS is the best motto!