Your starter could be going out, or gone. When my starter went out it would just do one "click" when I turned the key as well. Replacing it is not hard at all, 3 bolts if I remember right.
Make sure you take your old one with you to get it tested before you buy a new one. Only take the starter out to get it tested if you have tried everything else, but to me that sounds like what the problem is.
1990 Ford Probe LX 3.0L V6 :s6:
SEE IT HERE:
1989 Mazda RX-7 Turbo II 13B-T "FC3S"
Turbo RWD Rotary Powa!