As a new vapor starting out I can understand how it would be so difficult to decide on what to buy. You came to the right place. First off, there is all kinds of information here to help new vapors if you look threw the help threads....on top of that keep asking questions like this and we are all happy to do our best to try and help you out.
If you have a little Istick mini and it's always turned up to full power you might want to get a bigger Istick as well....the mini has only 10 watts of power which isn't that much, there is also a 20, a 30, and a 50 watt version. I suggest getting at least the 20 watt if not the 30....they're fairly cheap and reliable and excellent beginner mods that will give you plenty of power for your tank and open your options up to what kind of tank you want to use in the future as you vape more and get more comfortable with it.
As for a "cartridge" you're asking about....actual vaping cartridges are very old technology and other than some gas station and cheap or disposable gas station and "cig-a-like" type vapes they are not commonly used anymore. Clearomizers are pretty much the norm these days and they are the tanks you've been getting....I am going to go out on a limb here and assume you've been getting the cheap plastic disposable ones that you can't change the coil out on and just throw away when they where out.
If you want to keep just using the little Mini you have for now I suggest getting an Aspire Nautilus Mini and a spare back of Aspire BVC style heads so you have spare coils. That's about the best tank you can run on a Istick mini and will be your best bang for the buck IMHO. Coils are mass produced and depending on how much you vape and your liquid flavor your coils are going to last an average of two weeks...sometimes you get a good one that lasts a month, sometimes you get a bad one that only lasts a couple days....it's the luck of the draw since they're mass produced by the millions in a factory in China sometimes a bad coil slips threw. But you should be good to go for awhile with a Nautilus Mini and an extra pack of BVC coil heads....I also recommend you get either the metal tank or the metal and glass tank as the glass it comes with is somewhat fragile...it won't shatter if you drop it on the carpet or if you're lucky on the tile but they're kidna fragile and the metal tanks are a solid investment.
you can get the tank online fairly cheap as well as the coils . You can get a Nautilus Mini, a five pack of coils and a spare glass tube from Surevapes at the link here for 47 bucks including shipping. I really like this shop, I've ordered from them several times and they're one of the few shops that has free shipping on all orders over 10 bucks instead of 35-50 if not more...and that would be if the other shops even offer free shipping at all. Give it a look. It doesn't hurt to peak...you can try 101vapes as well...I tried to look it up but for some reason I get to their site right now. Best of luck, happy vaping...and keep the questions coming...we'll get you taken care of
https://www.surevapes.com/aspire-nautilus-tanks.html