Before starting promoting your app there are some steps that should be undertaken, especially if you don't have enough budget at the beginning.
You can submit your app in some free resources and directories that will help you find first customers and increase brand awareness.
Here's a good list with such resources to begin with: 50+ App Promotion Resources for Appreneurs
For app promotion, I highly recommend you boost your app ranking in the app stores, because about 65% users discover new apps by searching keywords in the app stores, and most of the just browsing the to 20(even less)apps, then the higher your app rank in the app store, the more downloads your app will get.
And the most efficient way to boost app ranking in the app stores is buying keyword search installs, which can boost your app ranking instantly, about 8~10 hours, it's really worth to have a try, save your time, monetize your app in a short time.
Promote app in related forums
There are many App forums which you can promote your app in there. You can ask people download your app and give you some advice to improve your app.
Collaborate with influencers
Most people like to follow others and be influenced by the words of some celebrities. Collaborate with these people can quickly build up your iOS apps’ identity and gain recognition in related fields. These people may include influential bloggers, newspaper journalists, and etc.
Get more app installs
Large amount app installs not only can help boost app store rankings, but also can make app rank higher in the search result. Data show that over 70% people download an app through searching in the app store. If your app have a large amounts installs, your app will come in the top few in the search result. You can ask your friends, classmates and families help, let them download your app.
Put your app on mobile app review sites
This method means that you have to find some reliable app reviews sites, putting your app on them to guide users to install your app. But you have to guarantee your app’s quality is good enough to be reviewed by audiences. When you reach out, you have to provide some support material of your app, like app links, summary, screenshots, video, etc. If your app received more positive reviews in these sites, you will gain a tons of traffic and installs.
Create blog accounts
Make full use of social media, create blog accounts and write some information about your app. Running these accounts, do not only write about your app, also write some funny jokes or stories to attract fans. Also remember often contact with fans, community with them, ask the advantages and disadvantages of your app, then you can improve it. And ask them help, let them advertise your app to their friends.
Searching some targeted keywords in Google and find the relevant blog about your app. Then, add a valued comment at below with valuable insight and your app link. Typically, this method is used in internet marketer space. But you can also use this method to gain high-quality relevant links and traffic to your app and landing page.
Gain positive reviews and 5 star ratings
App reviews and ratings play an essential part in app promotion, which can affect your app installs and ranking in App Store. Thus, to promote an iOS app, getting more app high-quality reviews and 5 star ratings are necessary for app developers. You can ask your friends and relatives’ help, exchange reviews in some related forums or incentivize users to give positive reviews and high ratings. But the most effective way is to buy app reviews and ratings from a reliable company which can provide real users’ reviews.