This passage occurs in the Epiphany season of the Revised Common Lectionary (Year A).
As usual, the Greek makes things more exciting! First, the word for "serve" in 6:24, as "cannot serve God and Mammon" is not serve like "wait on the table" but serve as in "be a slave to." Second, Jesus gives a number of commands throughout this passage. The tenses of the commands (for which I devote an entire section) highlight Jesus’ point. A brief review: Jesus tells his disciples to look up into the sky using an aorist command; he tells his disciples to seek the Kingdom of God using a present tense command. The one is a simple request; the other is a constant task. The translations generally do not capture this distinction.
δουλευω ("serve" or "be enslaved to," twice in 6:24): This word does not simply mean "serve" as "I painted a wall for a poor person's house." It is more akin to slavery and servanthood. "It is not possible to be a slave to God and Money." Saving "serve" allows us a bit more control, I think, than Jesus implies. The word for serve as in clean-up/pitch-in is διακονεω.
μαμωνα ("mammon" or "money," 6:24): This word is not a Hebrew or Greek word, but is Aramiac, meaning wealth or property. It it not found in the OT; it is not picked up in any of the NT letters. It appears a few times, three times in Luke and once in Matthew. I guess it is a deeper question -- is Jesus trying to personify money here or not? I would suggest so, based less on what the word Mammon actually means, and more within the context of serving a master.
ολιγοπιστοι ("little faith," 6:30; appears in a few forms in Matthew's Gospel, also 8:26,14:31, 16:8, 17:20); This is a "pet" phrase of Matthew. The question is -- is this a rebuke or an encouragement? Of course, there is some element of rebuke, but perhaps there is an element of encouragement. Even being of little faith is better than being of none! In fact, if you compare the stories in chapter 8 and 16 from Matthew's Gospel with their Marcan parallels, you will see that Matthew adds this word into to soften the words of Jesus! (Or Mark excludes it). It is a humbling reminder that Jesus teaches us, not simply when we are wise and have full faith, but even when we have little faith.
προσθειναι ("add" an aorist infinitive of προστιθημι in 6:27; also in 6:34): Jesus here makes the point that worrying will not add an inch to our lives but seeking the kingdom of God will add all these things to us!
1) What does that refer to?
In English grammar, a pronoun needs an "antecedent," ie, what it is playing the roll of pronoun for. For example, "She and her sister are nice. I like her." The "her" doesn't really have a clear antecedent and so it is hard to understand. In this case, Jesus says, "Seek the KoG and its righteousness and all these things shall be added unto you." The question is, what are these things? Righteousness or clothing and wealth? (Kind of like King Solomon, who, in asking for wisdom, gained wealth).
2) Tenses in commands: Aorist vs Present
Jesus gives a number of commands in this section. Commands can also be called imperatives. They provide a helpful way to understand how tenses function in Greek commands. In English, we really only have present tense commands: "Go!" or "Help!" In Greek, however, the command can be given in either the aorist or the present tense. This impacts how the verb should be translated.
The aorist is used for a simply command, like a "Do this now" sort of thing. For example, εμβλεψατε ("Look up in the sky!", 6:26) or καταμαθετε ("Consider the lillies", 6:28). An aorist command requests a specific action to be taken right now.
The present tense is used for a command that requires continued action. For example, ζητειτε ("Seek the Kingdom of God," 6:33). Jesus wants his followers to ALWAYS seek the Kingdom of God. While the "continuous" nature of present tense can be sometimes overstated, the present imperative strongly suggests a continuous action.
In English the distinction between these two tenses is often overlooked. In this case, a fair translation would be "always" or "continually seek the Kingdom of God." (Whereas you don't always have to look up into the sky)
With negatives, it is a little more tricky. I confess, I get them confused! Jesus uses the same verb here in both the negative aorist and negative present command forms, so this will hopefully clarify.
Jesus uses the negative present imperative in 6:25: μη μεριμνησητε. In this verse, he is telling the disciples a forever command: "Do not ever worry about your life." The implication too, with a present tense negative imperative, is that the listener was in fact doing this action. For example, angels often have to tell people μη φοβου. (Present tense of "fear") We translate this "Do not be afraid" but it would better as "Stop being and continue to stop being afraid." Or perhaps, more poetically, "Do not fear."
Jesus then switches to a negative aorist command in 6:31 μη μεριμνατε. This verb is actually an aorist subjunctive. (Why? Well, I don't know the deep reason, but the basic reason is that negative aorist prohibitions take the subjunctive mood.) In this verse, Jesus is telling the people not to ask "What shall I eat?" While this could also be considered a permanent command, Jesus uses the aorist here because he was neither suggesting that the disciples were specifically doing that at that moment nor was he suggesting that this would be their constant question. Lastly, Jesus tells the disciples (again using a negative aorist subjunctive) not to worry about tomorrow. The use of the aorist is almost humerous here. It is almost as if Jesus is saying, "For today, I tell you, don't worry about tomorrow." The command is not in the present tense because Jesus doesn't want them to think about their actions for tomorrow!
Final note: Sometimes trying to figure out why a particular author used a particular tense is challenging. This is certainly true with negative aorist prohibitions! They could easily have been present tense! Sometimes there is a bit of sloppiness in the tenses. However, when you see Jesus using a present tense command for seeking the Kingdom of God, you can know that Jesus wants them to do this all the time!