Hey connor123, you can sign up to a new plan online or through your My Account if you know what phone you want. If you didn't want to get a phone with the plan and just wanted to keep using the one you have now, you can check out the SIM only plans available here.
You've mentioned sharing data. Am I right to think that you're wanting to end up with two phones using the data pool? If that is the case, both services need to be connected to the same billable account number. This is how Optus recognises which phones are eligible to share your data. If both services are currently with Optus, you can consolidate them onto the same account via customer service.
Just to clarify Conner,
If you wanted to use own/two phones then you would need to get two Optus plans. You can't just select a single plan and have two phones use it together. As @Shae says, any new phone at the same billing address will automatically be able to share its data allocation.
Some people get a 'main' phone with the data they want and then put a second phone on the smallest/cheapest Optus plan. That way you can share the data but not pay as much. although its not a big saving:
3Gb + 15Gb plans = $70
15Gb + 15Gb plans = $80
So if you do want two phones then you don't have to wait until June. Just pick a plan and maybe a phone and get it and the data will be pooled. You should also check that Optus puts your old phone down to equivilent SIM only prices when your contract is up (e.g. $40 for a 15Gb plan)