There are a lot of ways to share your Sketch project, it depends if you want to share your prototype with the client or hand your deliverable to the developer.
I recommend using Invision, it is an online tool, you don't have to download any software, you just share your project with a link. With a free account it allows just one active project at a time, but if you want to upload a new project you can just store the old one. Invision is a great tool for both clients and developers, who can see the interactive prototype with transitions and animations, make comments, and view the code with inspector.
Another great tool for handing the deliverables to developers is Zeplin.
About Flinto, to share your prototypes you can record the preview window and share the mov/gif file, or you can share the flinto file to a phone and open it with the Flinto viewer app.
There are a lot of transitions that can be done with Sketch plus the plugin Craft, maybe you don't even need to use another tool like Flinto.
To learn about Sketch there are a lot of youtube channels I can recommend: Sketch Together, Joseph from LearnSketch.com, Jesse Showalter, CharliMarieTV, The Futur.
Hope this was useful! Keep on learning!