hi, i'm definitely not a plato expert, but what i have figured out is that you need to read lots of plato's dialogues to get into him. there's not a core dogma that can be summarised and that he's padding out into dialogue - if there was there might be some late dialogue where he finally decided to lay it all out, but it doesn't exist. the ryle book i mentioned above is one of the books that helps you see this is not right, but mostly you get this just by reading him.
as to where to dive in, i would pick out something from each of his phases, to see his complexity, and then go into each of them further. i'm still doing this.
for eristic, ie if you want socrates being super crafty and dragging people into confusion, you could start with book 1 of the republic (the rest of the book was probably written later for a different audience) - or if you want a bigger one, i really like gorgias, or in between the euthyphro is one of my staples, a full dialogue but short.
then for the non-eristic phase, ie if you want the deeper stuff, theory of forms and all that, phaedo is great, and the rest of the republic.
then if you want to see plato post-sicily, try the philebus, and then to see plato fully destroy the theory of forms and get into hardcore dialectic, read the parmenides.
another completely approach, one i've taken before, is to select dialogues based on number of words, so you get through lots quickly. ion is a good read, so is laches, etc. if you could clock up a large number of dialogues through reading lots of short ones and a couple of the bigger ones, over his different periods, then you would have dug into plato pretty well i think.