The mantra is: plain squares are good, plain squares are essential. Don't worry - dive straight in. The value of plain squares is that they can go with everything, and they can be used for decorative motifs when a particular theme has been requested, So they are great.
I would start with a garter stitch square (every row knit, easiest thing there is) and then you could do a framed stockinette square, basically like this:
first five or so rows - knit (to make the bottom border) row 6 - knit 3, purl to 3 stitches before then end, knit 3 row 7 - knit all stitches
repeat row 6 and 7 until the square is big enough, then do 5 more rows all knit. Voila. It's stockinette in the middle (alternate rows of knit and purl to make a smooth surface) with a garter stitch border to stop it from curling.