The classic 1970s-style poncho pattern is made by sewing together two rectangles. There are diagrams all over the internet for how to do it (one of which I've shared below), but I want to share some tips for designing your own poncho that go beyond making the shape in question.
As with any project, you will need to know your gauge and relevant measurements. For your gauge, make a swatch using the yarn, needles or hook, and pattern stitch you want to use. When you have a swatch that feels like the fabric you want to make and felt relaxed to make, measure how many stitches you have per inch. The number of rows per inch isn't all that important for this kind of project.
Next you need to measure the person who will be wearing the poncho. If you can't measure that person, you can use an online measurement chart to make an educated guess. For a poncho, the width of each rectangle is the length from the neck down to the point on the arm where you want the poncho to end. If you plan to put some kind of border around the finished poncho, then you'll want to subtract the depth of that planned border from this measurement. This measurement multiplied by your stitches per inch is the number of stitches you want to use, although, you might have to adjust that number a little to be an appropriate multiple for your pattern stitch.
The length of each rectangle is that arm measurement you just figured out PLUS half the head measurement of the recipient. Again, online measurement charts are handy if you can't measure the real person.
As an example, I recently crocheted a poncho for my daughter. Her arm length from the neck to the wrist was 14 inches, and I estimated her head at 20 inches around (I purposely made this poncho a little big on her, so she can grow into it). Therefore I made the rectangles 14 inches wide and 24 inches long (14+10).
Once you have made the first rectangle, you can save yourself some sewing later by picking up the stitches for making the second rectangle along the side of the first rectangle.
If you are crocheting, you can crochet the second seam, rather than sewing it. You can also avoid a hard chain edge along the beginning of you rectangle(s) by using a foundation stitch.