To make a good cover, you need good apps. I've searched a lot to find good apps and now that I found some that are really helpful I will help you find them too.
1. Superimpose
Superimpose is actually my favourite app to make covers. I will use this app for most of my tutorials. Unfortunatelly, this app cost money and in the first place I only bought it because of voidkira. It costs 99¢ and if you are canadian -like me- it will cost you $1,29 I think, I don't really remember but it was something like that. I don't regret buying it though.
2. Phonto
Phonto is an amazing app for fonts and it's free. I always use it and I highly recommend it to whoever want to make great covers.
3. Polarr
Polarr is an amazing app for filters. There is a lot of filters that the app already have and you can also make some yourself. I'll make a tutorial about filters you can make yourself.
4. Picsart
I started using this app like two weeks ago. I use Superimpose more that Picsart but I'll try to make tutorials for this app too.
5. Pixlr Editor
This is the app I'll use to make covers on the computer. It's completely free but I won't be posting tutorials for that yet since I just got my laptop.
6. Snapseed
Snapseed is a good app for filters and I recommend it because it's really simple to use.