I know that Dragon Ball is a very long saga.
- Which series should I start with?
- Is there any of the new ones the reboot of the old ones?
- If I start with Dragon Ball Super, will it be continuing the others, or is it a reboot of the first one?
If you want to start with the beginning, start with Dragon Ball. If you want to start with the most popular segment (probably the most entertaining segment, but this is a matter of opinion), start with Dragon Ball Z. Many people haven't seen Dragon Ball and yet they understand most of what follows it without watching Dragon Ball.
I don't know if Dragon Ball Z Kai counts exactly as a "reboot", but it is a modified version of Dragon Ball Z, in HD, with new color, new lines, new backgrounds, re-recorded voices and sound effects, new openings and endings, new background music, deleted scenes and episodes, etc.
Dragon Ball Super is a continuation of Dragon Ball Z and Dragon Ball.
Dragon Ball Super is the latest iteration of Dragon Ball (as of current writing). I recommend starting with Dragon Ball, followed by Dragon Ball Z and then watch Dragon Ball Super if you wish to watch all the episodes in order and in succession.
There is also Dragon Ball GT which isn't canon with the original series. You could perhaps consider it as something which happens in an alternate timeline and watch it after Dragon Ball Z. Please note that Dragon Ball Super is the actual continuation of Dragon Ball Z.