The thirteen epistles that are almost certainly written by Paul, in order, are:

Epistle to the Romans

First Epistle to the Corinthians

Second Epistle to the Corinthians

Epistle to the Galatians

Epistle to the Ephesians

Epistle to the Philippians

Epistle to the Colossians

First Epistle to the Thessalonians

Second Epistle to the Thessalonians

First Epistle to Timothy

Second Epistle to Timothy

Epistle to Titus

Epistle to Philemon

although some theologians doubt that some letters (eg Ephesians) are written by him as the style of writing is different from the others.

The fourteenth letter, the Epistle to the Hebrews - is unlikely to have been written by Paul. He doesn't claim to have written it nor is the style of writing his. Pauline authorship is rejected by modern scholarship and even in the early church its authorship was debated.

Epistles to the Thessalonians written from Corinth. Epistles to the Corinthians. Epistle to the Galations. Epistle to the Romans written from Corinth. Epistle to the Philippians. Epitle to the Colossians. Epistle to Philemon. Epistle to the Hebrews. Epistle to Titus.

Chapter 2, verses 1-2 of 2 Thessalonians talks of rumours that the Second Coming of Jesus was about to occur. Verse 3 assures them that it was not to be. If Paul really wrote 2 Thessalonians, this reassurance would be a good reason for him to do so, but scholars are reported to be almost unanimous that Paul didd not write 2 Thessalonians - it was written in his name later in the century.

The general consensus of scholars is that 1 Thessalonianswas the first New Testament epistle written by Paul. However, it is not universally agreed that Paul actually wrote 1 Thessalonians. If this is not really an epistle of Paul, then Galatians is thought to have been the first New Testament epistle written by Paul.