So odd question that I'm sure google could answer, but I want to make sure with a native speaker here. I am learning Korean to make a speech in July, not to be fluent. It is for people from Seoul, what should I be learning? I just want to make sure I don't learn a dialect or words that people from Seoul would not understand.
We call it "standard Korean" not "standard dialect." 서울말 is standard. All the Korean sentences and other stuffs on your Korean textbook are written in standard Korean. When you speak in standard Korean, everyone knows what you talk and mean. I dont think you need to learn different dialects. :)
서울말 is the standard dialect that is used so you won't have any problems.
Agreed with the other comments. The good news is that almost every learning material you come across will be using the standard 서울 dialect anyway, so I wouldn't even worry about it.
The 서울 dialect is probably the gold standard, but you'll probably be fine with anywhere as long as you don't learn too much slang. The major difference between dialects is informal verb endings and some pronunciation of certain words, so as long as you pronounce mostly how it's spelled and stay away from things like "뭐 하노?" and "뭐뭐 하는디" you should be fine in 서울.
I'm not a native speaker but I can tell you the Seoul dialect is what you should be learning.