I need to compare/sort two text files

this is the scenario. I have File1 and File2 and i like to have the outcome in File3. I'm kind of new to Linux, but so far ive tried to use sort, diff, and comm. but no luck so far. File1.txt File2.txt > File3.txt File1.txt RB0009 8,89 RB0010 5,67 RB0015 4,32 RB0027 6,56 File2.txt RB0009 8,89 RB0010 5,67 RB0015 4,32 RB0027 6,56 RB0033 9,78 File3.txt RB0009 700111i 8,89 RB0010 700092i 5,67 RB0015 700148i 4,32 RB0027 700123i 6,56 help would be much appreciated.