There are several knots that every cruiser should know. The basic ones are as follows.

1. Bowline
2. Cleat hitch
3. Half hitch
4. Reef Knot
5. Sheet Bend
6. Clove hitch
7. Figure 8 knot

There are many other useful knots that are worth knowing. Take some time and get a book or look these up on the internet and most importantly practice them. Of of the best resources that I know of is Ashleys Book of knots. You can find a downloadable version of this for free on the internet. There is much history and commentary about the thousands of knots that are listed in it.