Rerolling
Rerolling in Uma Musume is extremely fast, as you don’t have to redownload the game and can skip the tutorial after the first time, so it’s highly recommended.
When rerolling, you should spend everything on Support Cards. They are what determine the power of your account. Even the most meta umas can’t perform well in PvP with Rare cards. If the banner uma is someone you really like, you can try to get both it and a good starting card.
If you don’t get what you’re looking for, return to the title screen and delete your account data from the menu, then you can start again.
High Effort, High Reward
In an ideal world, you will get the banner card to 1LB or more through the initial pulls, which takes about 20 tries on average. Then, farm through runs to clear out all the races (you get Carats for the first time you win each one) and complete the various missions, getting enough Carats to reach 200 pulls on the banner. You’ll hopefully MLB the card in this process, or at least 3LB. If you don’t, you might restart from the beginning and try again.
This is easier during free pull events, of course. This process takes a long time and if you’re new to the game, it’s difficult to clear out those races, but if the banner is good, like Kitasan Black, it will set you up for success for a long time. Speed and Wit cards are the best targets for this. To estimate how many accounts it will take, use a Binomial Distribution calculator. p is 0.0075, n is how many pulls you start with, and x is how many copies you want to start with. P(X>=x) is the chance of succeeding. For example, starting with 3 copies in 60 pulls is about a 1% chance.
Later on, the game adds crystals you can use to limit break SSRs of your choice, so even if you end up with 3LB, you can finish it up with that.
Low Effort, Good Start
If you don’t want to put in that much effort, you could aim for some cards that are good with few copies. Of the cards currently in the game, Super Creek, Tazuna, and Fine Motion are the good targets. I would try to start with at least one Super Creek, and if you get any more of those three cards, that’s a bonus. Super Creek remains usable basically forever, while Tazuna is good in the first three scenarios, and Fine Motion is a strong Wit card even without limit breaks.