I would get the regular size Boge cartos to start. They hold about 1.2ml of juice so it will still be alot more than a cartridge and you wont have to refill as often. The Boge cartos are very popular and work well. They will be good for you to learn on, and a lot of vendors sell them. Most vendors have the 2.0ohm boges and the 3.0ohm boges. The 2.0ohm are considered "low resistance"(LR) and the 3.0ohm are "standard resistance"(SR). Most people prefer the LRs. Low resistance cartos will give you a warmer vape and more throat hit (TH).
I buy all my cartos from Crystal Clear Vaping (CCV). They have good prices and free shipping which is nice when you just want to order some cartos. Also CCV gives you a free syringe with all carto purchases and if you buy a 15-pack you get a syringe and a free drip tip. They sell the Boge 2.0ohm and 3.0ohm cartos.
You can use the syringe to fill them or there's the condom method, and just dripping into the top of them. I use a syringe bc it's very quick and easy and doesn't make a mess. The first fill on any carto is very important and a syringe ensures that it's done correctly because you push he needle all the way to the bottom and full from the bottom up. You have to be careful if you drip fill bc you have to let the juice settle to the bottom. If you don't wait long enough or put enough juice, you can ruin the carto and you will get that nasty burnt taste. Depending on what juice you use and how thick it is, it can take awhile for the juice to settle.
That's why I recommended the Boge cartos to start bc they are easier to fill and they will be easier for you to learn how to use cartos properly.
Then once your comfortable with cartos, you might want to then start using a larger carto that will hold more juice. I have an EPower which is very similar to a Riva and I use the "ressurector" cartos from CCV. They are 1.7ohms, so they will give a slightly warmer vape than the Boge 2.0ohms. They also hold alittle over 2ml of juice so they last a long time before they need a refill. Since they do hold more juice, they are alittle harder to fill and if they arnt filled correctly you can easily burn them. A syringe does make it pretty easy though. And I def don't recommend drip filling the Ressurectors bc they hold quite a bit. Dripping into them takes quite a while for the juice to settle and def longer than the Boges. It's fairly easy to burn them bc you might think you put enough juice, but it could still be dry where the atty is. The Ressurectors are awesome though and work extremely well on 3.7V PVs like the Riva, EPower, kGo, etc. I would get comfortable with the Boges first and then maybe try a bigger carto like the Ressurectors later, but that's up to you.
Goodluck!
You'll be a vet soon enough.
