This is a list of the tools you will need to start modifying KOTOR files
KOTOR Tool by Fred Tetra
download
The swiss-army knife of KOTOR modding, and the first thing any KOTOR modder needs. View and extract all game file types. Also includes a built-in GFF Editor, 2da Editor, Script Editor and DLG Editor (k1 only) which are all serviceable.
KGFF Editor by tk102
download (unofficial tool archive)
Edit files saved with BioWare's Generic File Format. This covers a broad range of game files - everything from characters to stores to triggers to journal entries. ARE, GIT, IFO, JRL, PTH, UTC, UTD, UTE, UTM, UTI, UTP, UTT, UTS, UTW are all examples of GFF files.
ERFEdit by stoffe (updates by FairStrides)
download
Pack and unpack the ERF and RIM archive formats, including the related MOD, SAV, and HAK formats. ERFEdit can extract resources from any of these files or create them. If you're editing an area or creating your own, you'll want to use ERFEdit to package all its contents in a .mod file for release.
2DAEditor by VarsityPuppet
download
Edit any 2-Dimensional Array (2DA) file. 2DA is a fancy term for a spreadsheet and a lot of critical files are 2DAs because spreadsheets are handy - things like character appearances, player classes, and item types. 2DA Editor has a few nifty features that KOTOR Tool's editor lacks, like filtering search terms and copy/pasting cells.