1 Μακάριος ἀνήρ, ὃς οὐκ ἐπορεύθη ἐν βουλῇ ἀσεβῶν καὶ ἐν ὁδῷ ἁμαρτωλῶν οὐκ ἔστη καὶ ἐπὶ καθέδραν λοιμῶν οὐκ ἐκάθισεν, 2 ἀλλʼ ἢ ἐν τῷ νόμῳ κυρίου τὸ θέλημα αὐτοῦ, καὶ ἐν τῷ νόμῳ αὐτοῦ μελετήσει ἡμέρας καὶ νυκτός. 3 καὶ ἔσται ὡς τὸ ξύλον τὸ πεφυτευμένον παρὰ τὰς διεξόδους τῶν ὑδάτων, ὃ τὸν καρπὸν αὐτοῦ δώσει ἐν καιρῷ αὐτοῦ καὶ τὸ φύλλον αὐτοῦ οὐκ ἀπορρυήσεται, καὶ πάντα, ὅσα ἂν ποιῇ, κατευοδωθήσεται. 4 οὐχ οὕτως οἱ ἀσεβεῖς, οὐχ οὕτως, ἀλλʼ ἢ ὡς ὁ χνοῦς, ὃν ἐκριπτεῖ ὁ ἄνεμος ἀπὸ προσώπου τῆς γῆς. 5 διὰ τοῦτο οὐκ ἀναστήσονται ἀσεβεῖς ἐν κρίσει οὐδὲ ἁμαρτωλοὶ ἐν βουλῇ δικαίων, 6 ὅτι γινώσκει κύριος ὁδὸν δικαίων, καὶ ὁδὸς ἀσεβῶν ἀπολεῖται. 
(1:1) How happy is the man who does not go in the counsel of the godless and who does not stand in the way of the sinful, and does not sit in the seat of pestilent persons, (1:2) but rather upon the law of the Lord is his desire, and upon his law he fixes his mind day and night. (1:3) And he will be as a tree which have been planted along the springs of water, which gives its fruit in its season and its leaf does not fall away, and everything whatsoever he may do, it is made to prosper. (1:4) No so for the godless, no so, but rather they are as the powder which the wind casts away from the face of the earth. (1:5) For this reason the godless will not rise up in the judgment nor sinners in the counsel of the righteous, (1:6) because the Lord knows the way of the righteous, and the way of the godless will be destroyed.
I normally (normally, I haven’t done this in months!) include a polished translation, but I’m terrible at poetry, so there is no polish here.
A thing to remember when read translations like mine is that when I’m translating “κύριος” as “the Lord” despite its lack of an article it’s because I’m following English language conventions. The Greek word is used to translate the divine name. So it’s really a circumlocution for a personal reference to God. It might even be better to translated it into English as “YHWH.”
In (1:1) we see a reference to “pestilent persons.” In the Hebrew Old Testament, the word is “scoffers,” a particularly vicious type of sinner. The translators picked a harsh word for these types of people, but it works. “Pestilent persons…” people who spread their impiety or bad attitudes like a disease. It’s frightening and elegant. Anyway, the Psalmist is warning us in a roundabout way (happy is the man who…) to avoid the fellowship of such people. There are obvious exceptions to rules like this. Jesus spent time with sinners, even such as wanted to kill him. But the idea is in terms of influence. The average person takes on the character of their closest pals (think Jimmy Olsen and Superman). If you sit in the seat of scoffers you’re either sitting in their homes or, worse, sitting in their position to do their scoffing. When I hear Christian academics making fun of their brothers and sisters for unenlightened views (there is, of course, a place for mockery in moral instruction), I cringe as this Psalm comes to mind.
In (1:2) we have the contrast that shows how the previous section is not a call to avoid sinful people all together, but to avoid letting them be the prime influence in your life if you wish to be happy. Instead, one should meditate on the law of the Lord. In the case of the Psalm this clearly refers to the first five books of the Old Testament. For Christians this meditation includes the Old Testament but extends to and shows a preference for Jesus, his way of life, and his teachings. Paul talks about this in 2 Corinthians 3:18-4:5 and Colossians 2:1-7 & 3:1-4, Peter talks about it in 1 Peter 2:21-25, and John says it in 1 John 2:6.
In (1:3-6) the Psalmist uses an arboreal analogy to make his point. The one who meditates on God’s law will be like a healthy tree, happy for two reasons. One because it is nourished and secure and two because it bears good fruit. Similarly, the righteous person has joy from knowing/delighting in God (1:2) and being known by God (1:6), but the righteous person has also learned the ways of success from the maker of nature and humanity and can therefore live with positive results in life. That this is the opening Psalm is telling because the “two-ways” paradigm exists throughout Scripture. It goes back to Genesis and is expressed in Revelation. But there are exceptions. Many times the wicked prosper at the expense of the good and many times the good can be crushed between the gears of historical circumstance. The whole book of Psalms acknowledges these exceptions, but it starts with the general rule and encouragement to goodness: the truly good are the truly happy!