暑い is (too) hot, talking about the weather. ex) I want to go out but it's 38 degrees hot (Celsius) outside. it's 暑い. 熱い is hot , talking about a sensation/generally. ex) Which do you prefer? Hot coffee or Iced coffee? Hot ,please. It's 熱い.

The comment by Kaede is well simplified so that you can understand. And let me leave further comments. 1) When you are in fever with somethig, 熱い is only available. ex: わたしは日本の着物について熱く語った。 　　　　I told about Kimono in Japan enthusiastically. 熱い恋に落ちる devote to love 私には日本へ行きたいという熱い思いがあります 　　　 I have a burning desire to go to Japan! ect...