Authy if you have an iPhone, or Google Authenticator on Android - necessary to use 2 Factor authentication

Blockfolio or Delta - to always keep an eye on your investesments, very usefully if you have altcoins around exchanges, easy way to check all in a round

Some coins have mobile wallets, i suggest dont use them, you can lose your phone easily and dont want to lose your crypto too.

If you are android user, be careful with fake apps, yobit and other sites are impersonated by fake apps to steal your money.

You will start receiving a lot of scam emails, always double check every email you got, dont click on any link, unless you are sure about the source,

coinbase, blockchain, and a lot of crypto related apps or sites are always impersonated by scammers.

I dont suggest trading on phones, if with big screens is hard to see the full perspective, with small screen is very very hard, and also not good for your eyes