"What software can I use to create maps for my RPG?"
It's a good question and one that has alot of different answers.
First I would suggest you get familiar with The Cartographers' Guild. This is a great resource for cartographers. Specifically, they have an entire sub-forum to discuss the various software that can be used to create maps.
Having spent some time there I decided to purchase Campaign Cartographer 3+.
Pro Fantasy create CC3+ and state on their web-site that they have led the field in map-making software for gamers for 20 years! It's a big call to make but lets you know instantly that this is not a new piece of software coming onto the market. This is established, with a fan base and community to support it. This is something I always look for with my RPG software as alot of software rarely makes it to the official launch phase.
This is particularly important for CC3+ as there are a HUGE amount of assets made available by the community. You just need to download them are start using them to create amazing maps! Check out the CSUAC and Dundjinni Archives if you plan on using CC3+!
Check out the video below to see the software in use. I've only been using it for a few weeks, so I have included some amazing maps below made by experienced users so you see can the real potential of this software.
Campaign Cartographer 3+ is designed to make overland maps.
Dungeon Designer 3 is an add-on to assist in the creation of Dungeons and Floor-plans.
City Designer 3 is an add-on to assist in the creation of cities and towns.
I don't own it yet but I'm keen as mustard to try out the Perspectives 3 add-on so I can make some isometric maps!
There's also quite a few more add-ons so be sure to check out the Pro Fantasy Website!