Are there any tips as to how to set the tightness of my stiches?
Good quality patterns, especially for clothes, have information about 'gauge'. These numbers will tell you the tightness that the creator of the pattern intended: how many stitches fit horizontally and vertically in a square of 10x10cm (4x4 inches) or 1x1 inch (2,5x2,5cm). Before you start working on the piece itself, you should crochet a sample: a small piece of fabric using the pattern. When you reach a bit more than 10x10cm in size, measure the gauge of your sample and compare with the pattern. Crocheting a sample is especially useful if the pattern calls for a certain brand of yarn and you want to use a different one.

The gauge is partly determined by the weight of your yarn and the size of your hook. All yarn skeins should have labels with information on suggested hook size and expected gauge. This page has a table of all commonly used yarn weights descriptions and their adequate hook size and gauge.

A gauge information may look for example like this:

Suggested yarn is the brand the creator used, but you should be able to use this pattern with any other Aran weight yarn (checking the page mentioned earlier you can see that it's also called Worsted, 4, medium etc.) and a 6mm hook. Your sample should have 12 stitches horizontally in 10cm (4in), but there is no information on vertical gauge. In this case you can just crochet 4-5 rows of single crochets and check if the tightness is good.