First of all, you have to study the MSDS of the strong acid first before working with it. Generally, the most useful action is to rinse your face with water. However, this will only make matters worse for strong acids like H₂SO₄. For this case, you have to wash the strong with a mild, soapy solution. If you are dealing with acids that do not react with water like HCl, then you just have to place your face under a stream of water for several minutes.