I’m happy that you started having interest on stamps! The stamp world is a magic one.
So your question is an interesting one. I’ve been collecting stamps since some years now, and it has been not a long time since I really decided what to collect. I think it is better to chose what to collect and not just to keep every stamp you find. That is for two reasons: first, having some spare stamps helps when you are organizing swaps; second, I think that keeping all the stamps does not make up a collection, but just an obsessive compulsive accumulation.
As I am Italian, I am collecting Italian stamps, but just up to the year 2000. Why so? Mostly because newer stamps are more expensive to obtain. I prefer used stamps for two reasons: first, they’re more cheap; second, I think a stamp with a postmark has much more things to tell you than just a new stamp alone. I may explore for example the journey it went through. I usually take them off paper, which is quite incoherent with what I just wrote about the journey of the stamp. I just think that off paper stamps look more neat in a collection. Anyway, when a find a special postmark, I make sure to keep it as well, so I cut around the paper. I keep them on paper as well when there happens to be self adhesive stamps. I try to soak them on water, but if I see it doesn’t work I leave them on the paper, cutting around the postmark if it is an interesting one, else cutting close to the stamp edge. But as I collect only pre-2000 stamps, there are very few self adhesive ones. (lucky me )
I find it really interesting to study the history of my country through stamps. I am particularly happy about my pre-1900 stamps, when Italy still was a kingdom, and the stamps during the fascist era. Those years are a dark chapter in the history of Italy, and having the possibility to touch with hand the propaganda stamps or the celebrative stamps really helps me to dive into those sad years.
I’m collecting a particular topic as well, which is flowers on stamps. I started doing this because the just look really nice and colorful to me. I started accumulating pretty much every flower stamp, but right now I’m considering to make my collection more precise (remember the “obsessive compulsive accumulation” statement? ), for example by keeping only those stamps that look more “botanical”, so that have the Latin name of the flowers as well, and not just the picture of the flower. In this collection I am not strictly considering only used stamps. I just don’t care much. Obviously I prefer them used, for the reasons I stated about the Italian stamps, but if there happens to be a new emission of flowers on stamps and I manage a swap with someone living in the country of emission, I prefer to ask them to put the stamps new inside an envelope. I’m really scared that they could get damaged during the trip. So another thing which is different between this collection and the Italian one, is that with the flower stamps I try to keep up on date, so I constantly check whether there are new emissions and then try to arrange a swap. Let’s say Austria has new flower stamps: I just look at the Austrian postcrossers, browse them by the most active or the last seen on the site, and, if they accept direct swaps, I contact them. For the flower stamps, as I collect the more recent ones as well, I happen to have many FDCs and minisheets. But again, in this case seeing them beside the used ones does not make me uncomfortable.
I have been thinking about since some time about starting an airplane collection as well, because aviation is my passion and I would really like to study this topic through stamps. But I just started, so I have no more than 50 airplane stamps I think.
I hope I helped you! Have fun!