I want to develop an App in IOS where user connect to a cloud DB and access information. what tools should I use?

There are a ton of awesome ones out there, all have their own strengths and weaknesses. What's nice is that you don't have to rely on just one and can mix and match as you see fit. Since Parse shut down, I've used AWS Mobile Hub (Amazon), Firebase (Google), and CloudKit (Apple). Firebase is my favorite but with one huge downside of lack of asset hosting support. If you're trying to store large amounts of photos or videos for example, Firebase isn't your best bet. I've only ever used AWS to host files in conjunction with Firebase but it works like a charm. I have a love hate relationship with CloudKit. It has an absurd free tier as it grows with every user, something like 1TB capped, extremely generous from Apple, however, it's not as snappy as Firebase, only works on iOS (you could use it with JS if if you are web handy but no native Android), and the operational API is verbose to say the least. It's pretty foolproof and asset storage comes natively. Neither are exhaustively documented but well enough to get a gist. CK is newer and has significantly less SO questions. I also ran into a problem with App Review having an issue with CK in the review environment. 2 out of the 3 apps ended up sorting themselves out on second review but one, more complicated app, I ended up replacing the entire back end which was a pain. Firebase is also cross platform if you end up learning Java and Android although the syntax is slightly (and annoyingly) different.

Treehouse used to have courses with Parse but have been deprecated. I'm sure they are working on something new but TBH Firebase is simple. It's a Cocoapod install and then just read the Firebase iOS QuickStart. Ray Wenderlich also has a nice ~hour long tutorial here.