Firstly to be able to play with the selection of all 24 characters you will need to finish the story mode.
With this many characters to choose from it might not be easy to answer but during the story mode, you should have had ample time with most of the characters to see which one might the best for you. Each has a different style of play and matches certain players better than others depending on their own style. The important thing to remember is no character is stronger really than another, they are just different to play and can be just as effective in the right hands.
This means that each character has both strengths and weaknesses but some are easier to pick up and play with than some which have very specific move sets. This means playing with each character is important for the mainly 3 v3 battle which you can switch between, so learning more than one character is more beneficial to you.
To start it is probably best to use with SSJ Goku or SSJ Vegeta at the beginning of the game as they are quite well-balanced fighters. They have decent move sets which are fun to play and they are powerful which are easy to usually land on your opponent.