Before you even think about getting fish you need to buy the tank, a filter, heater, gravel, gravel vacuum, de-chlorinator and a couple of buckets.
Oh and the most important thing of all - a test kit for ammonia, nitrIte and nitrAte. Knowing your pH also comes in handy but ignore any advice from your local fish store to try and change it. Fish do fine in most pHs.
Then you need to set up the tank with the filter and heater, fill it up with de-chlorinated water and turn everything on. You let the tank run for a day to make sure everything's working. THEN the important stuff begins...
You MUST do a fishless cycle before adding fish if you want your new additions to survive.
First you need to know about the cycle...
Basicaly, fish eat food and produce waste. The waste contains ammonia. Ammonia is deadly. Luckily good bacteria grow in the filter and gravel in your tank that change this ammonia to nitrIte. NitrIte is also very toxic but, again, there are good bacteria that use this. They change the nitrIte to nitrAte. Finaly, this much less toxic nitrAte can be removed by you. Every week you have to syphon out one quarter of your tank's water using the gravel vacuum and replace with fresh, de-chlorinated water from the tap. This weekly water change dilutes the nitrAte and ensures the water remains safe for your fish.
So that's 'cycling'. Besides water changes, you'll need to clean out clogged filters every now and then. Usualy once a month is sufficient. What you have to realise is that the good bacteria that are so important for making the cycle possible grow mostly in your filter. Chlorine will kill them. Unfortunately, chlorine is found in tap water. That's why you add de-chlorinator to the water you are putting in your tank - it takes out this dangerous chlorine. Chlorine also harms fish. Anyway, because the bacteria will die if exposed to chlorine, you have to rinse out your filter media (the sponges inside your filter) in a bucket of old tank water. This is convenient to do half-way through a water change when you've removed tank water - just before you add fresh de-chlorinated water. Just thoroughly squeeze out all that gunk from your filter. Regardless of the manufacturer's instructions, you do NOT need to and, indeed, should NOT change your filter's media on a regular basis. Only if it's actualy decomposing should you even consider it - even then, only change one bit at a time.
Back to fishless cycling... You can do this most effectively if you use pure ammonia but fish flake food will also work. Just put a couple of flakes in the tank. Test the water daily. As the flakes rot, ammonia will be produced and bacteria will start to grow that will change it to nitrIte and so on. Monitor the process using your test kit. As there are no fish in, just add more flakes when the original ones have mostly gone. Keep this up and test every parameter daily until you have 0 ammonia and nitrIte and high nitrAtes. At this point, vacuum the gravel thoroughly and do a 90% water change. Then go get your first fish.
Seeing as you seem to already have a tank, I assume you may already know some, if not all, of what I've just typed. Still, it's better to tell you again than find you didn't know and cost the fish their lives. Regardless, the fact that you already have a tank means you can speed up the process of fishless cycling the tank. Just move a little bit of gravel from your established tank to the new one when you first add the flake food. This will introduce some good bacteria and speed everything up. Still test daily etc.
When it comes to adding fish, none of what you suggested as species you'd be interested in would work except the betta... However, not 2. Either get a single male and get a snail and some shrimp for tankmates or go for a female community of 4 female bettas. 2 males, 2 females or a male and female will not work - in other words, no combination of 2 bettas would work in this tank.
If you decide against the betta idea, a group of guppies is an option. Alternatively endler's livebearers or platies. Regardless, you should have no more than any combination of 5 of these livebearers in the tank (assuming it's a 10 gallon). You are also best off going for all one sex so they don't multiply (you'll have to find homes for the babies then) and so you don't need to worry about ratios. With both the guppies and endler's I'd go for all males. With the platies I'd go for all female. There's a pinned topic on sexing livebearers either in the FAQ section or the livebearers section. It's very easy so worth a look. Basicaly the anal fin in males is tube-like while in females it's fan-shaped. It's obvious when you compare them. Again, sorry if you already knew this. Oh and don't mix any of these with the betta.
Alternatively, maybe you'd like to get some pygmy cories. A group of 7 would be very endeering in a 10 gallon. Or you can get 5 and put them in with a single male betta. They usualy get along. Don't get any other cories though - only pygmies are small enough of the common varieties. The rest are too big and active.
If you are willing to do some extra research, dwarf puffers (maximum of 2) OR certain killifish OR some shell-dwelling cichlids would also work in a 10 gallon and make very interesting inhabitants.
Anyway, good luck but make sure you research the fish you are interested in BEFORE buying them. Also make sure you've fishless cycled the tank BEFORE buying the fish. These two things are incredibly important.
