Here is the code that aims to figure out first letter of each sentence in a string of text:
StringTake[StringTrim[StringSplit[WikipediaData["computers"],"."]],1] Image may be NSFW.
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It will help to know what it means by the error in red displayed:
StringTake:Cannottakepositions1through1in.StringTake:Cannottakepositions1through1in.StringTake:Cannottakepositions1through1in.