No one has mentioned the ultimate parental accessory - the second pair of eyes.
Maybe the nanny counts as this anyway.
Also, think about child proofing your yard - specifically make sure they can't escape if you are all out in the back yard together. Make sure the fences are secure, there aren't any holes and that the gates are lockable (at least so that a child can't open them by themselves).
You are almost at the stage of meeting other parents via school, but in the meantime it may be worth seeing if there are organised groups of parents with similar aged children. For example - my wife takes our son to "playtime" at the local church on Mondays, where a horde of pre-school aged children cause havoc for a couple of hours, and the parents get to have a coffee/interact with other parents. All the children sit down and share fruit (each parent brings one piece and it is all cut up...). The older kids do craft.
Don't go too overboard on buying clothes/shoes - little ones grow so fast. Also, there is nothing wrong with buying kids shoes and clothes second hand - most are still in quite good condition. My wife regularly visits a local charity shop and also has found and sold some quite good things on ebay. (Note though - my son has also trashed some things that really are no longer sellable...).
Edit - forgot to say - congratulations! You are doing a most noble thing. As others have said, you will make mistakes - don't worry that is normal. If you feel like you want to learn more about parenting, I suggest pick up any parenting book and start reading, but don't think you have to follow the steps - any literature or course you read/do is a guide only - otherwise, use your common sense.
Also - make sure you pick up some child friendly pain killers - you'll understand the first time they have a howling fit that nothing seems to fit (especially the smaller one, whose communication is probably not great yet).