Sax players who are learning to improvise tend to give less thought than they should to what rhythms they are playing. So it will probably help if you get a recording of the song to play along with, and try to build phrases at least two measures long (even longer is better) where you are just playing one pitch, and where you are making it interesting by telling a story based on rhythm. What would a drummer play, if they were playing a drum solo, instead of you playing your sax solo?
Keep in mind that the audience listening to you is more likely to forgive and forget a note you play with the wrong pitch, and less likely to overlook a note you played at the wrong time.
Another tendency of those learning to improvise is that they think they need to play more notes than they actually do. Space between phrases is your friend.
While there is some technical stuff you will need to learn, such as which notes are in the chord, and which pitches are in the scale or key of the chord, remember that a good solo is like a conversation. In a conversation, using less words is often more effective than using more words, and your phrase #2 should sound like you know what the phrase #1 preceding it was (whether you or an accompanist played phrase #1).
