Hey Aim - First off - Welcome to the Board!
If you already have the boots then take them with you to try the fins on.
Fins come in 2 varieties Open heel and full foot types. Full foot fins just slip on your feet, no boots needed. They are great for sandy beaches and boats. Open heel varieties allow you to wear a boot inside so that you can walk to the water with some foot protection (very handy if you are walking over sharp rocks, etc..) I find open heel fins to be easier to put on and take off also.
If you plan on getting into scuba diving, and you don't want to spend money on 2 different pairs of fins, just ask for Scuba fins - they tend to be a bit larger and stiffer. While your muscles may have to get used to them, it doesn't take all that long (took my legs about 2 weeks/6 dives) before they were comfortable with the additional effort, plus my muscles got stronger as a result.
Then find a pair that fits comfortably, not tight or you will get foot cramps.
Going into your local dive shop they can help fit you for a pair and try several on. Plus show you all the variations and discuss what you will be doing.
For what it's worth here's a link to the pair I am partial to (but not everyone else is...lol)
http://www.kidspot.org/personal/scubaboard/fincomparison.html
Aloha, Tim