
You need to bear in mind that entries for Championship shows start to close TWO MONTHS before the show so you will be thinking of entering shows from the time she is 4 months.
Open shows are less competitive generally than Chasmpionship shows, and the best ones to start at if you can find some are the Companion/Exemption show. the judge may or may not know what they are doing at these but it is the practice for the dog that they are usueful for not gauging it's relative show merits.
No reason why you can't enter Championship shows right away also if you ahve been attending Ringcraft and learnt to handle well by then.
Always read the schedule definitions of classes adn rules section. Enter the lowest breed class available, and something like Puppy stakes or any Variety puppy. If it is an Open show perhaps enter the lowest two breed classes.
When your pup is older you might want to enter Puppy and Junior just to see how it will fare against older competition before it has no option but to go in that class.
I hae always found myself less nervous in the mixed classes because you don't feel fellow exhibitors are chaecking yoour dog out in the same way that you might feel self concious in the breed classes. It is also good for the dog to learn to show well among it's own breed and against other breeds.