Carlos, get ready to be overwhelmed with answers. If you are new to diving, the guy at the shop should be trained on proper fin sizing. When I worked in the dive industry I started with the boot as that would dictate how big the pocket on the fin would be. The boot is dictated by what type of diving, temp, feel and so on. If you are getting pocket fins, warm water diving, it is a little easier. You will get many opinions about what type to use from different people.
I like split fins. There are people that hate split fins. I am not a fan of the blade type of fins. My style of diving works well with split fins. There are also fin weight, fin color,fin buckle releases, type and ease of fin straps, fin bouyancy and of course calve strength for proprelling you. Always remember when buying fins: do not base them on comfort when walking around. These are not walking shoes. I can't tell you how many new divers base comfort on walking around the store in fins. Good luck to you Carlos! BTW, there is a good chance if you stick with diving you will have multiple pairs of fins depending on the needs of the particuliar dive.