Creating tables with the textile formatting used by Redmine is fairly easy though it is not listed in the help documentation link. It has been useful to me to create tables specifically in the wiki informational pages. Typically within the Redmine wiki I like to keep things simple so below I will only describe a couple attributes related to tables within textile formatting.
Table Example: three rows, three columns
Table Example: three rows, three columns with the top row as a header column
|_. one-one|_. one-two|_. one-three|
Table Example: three rows, three columns with top rows cells content centered, the second rows cells align left, and the third rows cells align right
|=. one-one|=. one-two|=. one-three|
|<. two-one|<. two-two|<. two-three|
|>. three-one|>. three-two|>. three-three|
Table Example: four rows, four columns with the top row spanning all four columns and the second row/first column spanning three rows.
There are some other attributes that can be specified as well including adding colors to rows. The information above was obtained from this site which also includes other textile table formatting examples. It can take a little while to become comfortable with writing pages in textile format but once you are used to textile it will become very easy to format pages for your Redmine wiki pages.