99 mustang problem nervous!?

the first thing you do, before anything else is drive the car down to a local auto parts store park, leave the engine running transmission in park and get out and check that the transmission fluid is not low, Usually it should be no more than a pint or 16 ounces low. You check the fluid when car is warm and running in park. If it says add, then put only a half a quart. You buy it in quarts, and you may need a funnel, so put some rags in the car and check for low fluid. If this fixes the problem, great, find out it there is a fluid leak somewhere is next. good luck.