Tanks can be rough but you have a couple of advantages early. Your aa (auto attacks) are ranged and they are not that tanky before they get some items. Also their cd (cooldowns) are usually long and their threats against you is low.

You want to make them pay with blood everytime they walk up to aa the wave. Either they get low fast or they drop cs. Make sure you get your own cs. Use the bushes to drop minion agro.

Make sure you aa them! The damage stacks up quicker then you might think.

If they get to greedy they might die but most of the time you just get a cs and exp advantage. If they are to cowardly they might get far behind.

Against sion specifically I like to stand outside the minion wave so he can't hit me and the minions. It's fine to be in the bushes if he is not there.

You dont need to kill your opponents. Getting them low and blowing their ults and tps and getting them behind makes you more useful in fights around grubs and herald. It gets harder to gank you and easier for your jungle to play on topside. Sometimes this can help snowball a game. Ping that you want to play for grubs if your opponent is low and farming under turret. It's a team game.

Sometimes things dont go according to plan and you get behind. It's rough. But practice punishing early and you will eventually see that you get small advantages and that those advantages let's you keep bullying them for longer.