One of the tough things about mages is that their auto attacks are very slow and very weak, making it difficult to CS without spending mana, especially if you get shoved under tower. It takes a lot of practice on each particular champion to know how low a minion needs to be for you to be able to kill it with just an autoattack, and how to set up minions for the tower to help you CS rather than hurt you. This is why many mages are looking to shove and roam, to not only prevent that from happening and force it on their opponent, but also have a bigger influence on the map.
You may be tempted to put stealth wards in the brush alongside the river. Those are wimpy wards. They only really show you what's in the bush, and if the enemy jungler is already there, there's nothing you can do to stop the gank. Put control wards there instead. Put your stealth wards further into the river, so you can not only see when the jungler is approaching your bushes, but also when they are taking the other paths around the river. If you can, look to push up and drop a ward into the chicken pit to keep eyes on the enemy jungler's clear and pathing.
I emphasize warding because mid is not a 1v1 lane. It's more like 1.5v1.5, because junglers love to come through mid for ganks. You have the jungle on both sides of you, so it's critical for you to track where the enemy jungler is so you can not only push up and get aggressive, but also keep your teammates safe from ganks.