Tip: See my list of the Most Common Mistakes in English. It will teach you how to avoid mis­takes with com­mas, pre­pos­i­tions, ir­reg­u­lar verbs, and much more.

Both “help someone do something” and “help someone to do something” are acceptable. The form without “to” seems to be more common in everyday speech than the form with “to” (especially in American English), but both forms are common in formal writing:

Some learners try to combine the -ing form we can find in other phrases (such as “go doing something”) with “help”, but this is unfortunately not correct:

Finally, there is one informal phrase in which we do use “help doing”, namely “cannot help doing”. If someone “cannot help doing something”, they cannot suppress the need to do it. For example:

This idiom means the same as “cannot help but do”—we could also say, “I cannot help but think about her constantly.”

By the way, if you haven’t read my guide on how to avoid the most common mistakes in English, make sure to check it out; it deals with similar topics.