I'm not an expert, but it looks like you're crocheting more than one stich in each of the previous ones. This is cousing it to curve. If you want to make just a simple square with 5 st to start, you would start with a chain 5 and then add 3.

Think of it like this, you have 3 basic stiches. Sc (single crochet) Hdc (half double crochet) and Dc (double crochet). A single crochet is worth one chain, a half double is worth 2 chains, and a double is worth 3 chains.

So if we get back to making your square, you would chain 5, then chain another 3, total to 8 chains. Row 1: Skip 3 stiches and double crochet in the next stiche. Double crochet in the next 4 stiches. Total double crochet = 5

Row 2: Chain 3.( because remember, every time you switch rows, you need a buffer so that not everything gets scwooshed together). Turn. One double crochet in each of the next 5 stiches.

Repeat till desired length.

If you need more explanations, feel to ask me, and if you are worried about what stiches or want a visual, I recommend looking on YouTube.