There's a Mac version now? Or is that merely the observation that the browser version also works on a Mac?
Girls aren't going to be very powerful until you can get them to seven stars, so it matters enormously that some are much easier to star up than others. The easiest girls to star up are the one star girls available in advanced exploration. You could readily get them to seven stars and have a viable team within a month. Two and three star girls who are available in training and ascension missions can also hit seven stars over the course of several months.
Certain girls can readily hit seven stars from their launch event. I got Ortylia, Summer Naladia, and Perille that way. But if you missed the launch event, then you missed it, and just ignore that girl. Most girl launch events don't give you a non-whale way to get them to seven stars.
Some girls are also a lot stronger than others. Those with a higher starting star rank tend to have higher stats, but that doesn't always make them stronger. Some natural two or three star girls are just terrible.
Elnora and Chharodi are the strongest of the natural one star girls, and are available in the advanced training at the end of regions 3 and 4. 30 shards per day from that could get them to seven stars in under a month, and that will get you a viable team quickly. Naladia (end of region 2) isn't nearly as good, but isn't completely terrible, either. Trisha (end of region 1) is awful. Ignore her.
The two star girls with training missions are Grace, Seleia, Leina, Eremia, Frey, Shalutta, Fealty, Remy, Sasha, Celstia, Lilian, and Rigg. Don't focus on powering up two star girls not on that list. Of those, Grace, Seleia, and Leina are probably the strongest. The two star training missions go in a four week cycle, with three girls available each week.
The three star girls with ascension missions are Charlotte, Isabel, Angela, Claire, Khalis, and Yjer. They are available for two weeks at a time, with two girls at a time. Those are on a six week rotation. Other three star girls are basically unobtainable outside of their launch event unless you're a serious whale. Charlotte is the strongest of those six and by far. Angela is the only other one whom I'd regard as worth getting, and even she is only really useful as a counter to Perille in arena. Khalis, Yjer, and Isabel aren't bad, exactly, but they're not any better than Leina, Seleia, and Grace, and they are more expensive to get.
That said, while you aren't going to fully power up and use most girls, you do eventually want to star them all up. Star rank bonus makes a huge difference.
When you reach level 20, you want to join an established league that is ranked relatively high. It doesn't have to be a top ranked league, but at minimum, you want a max rank fat cat so that you'll get 5x coins from missions. The top leagues have also maxed their fairy ranks by now. You don't need to get into the #1 ranked league, but even one that is ranked #20 on your server will be massively better than one that was just created.
Each league has a feast at some four hour period every day that will refill some of your AP, arena banners, and raid horns. Make sure that you get a league whose feast is at a time that you can usually make it. You only have to click at some point within the four hour window, not be there for the whole four hours. It would be better to be in the #20 league and make the feast than the #5 league and miss it.
You generally want blue or purple gear for your characters, and mostly purple. Purple gear comes from raiding. You'll naturally get a lot of blue crystals over time, so having one blue set of gear that you star up can get you a decent set of gear for one girl to supplement what you get from raiding.
The scarce resources for powering up girls are universal crystals and amethyst evolution crystals. You'll get other things like coins, experience books, and forging materials much faster. Always buy universal crystals when they're available in the noble shop, even if they cost diamonds.
My priority for spending diamonds is:
1) universal crystals in the noble shop
2) reroll league missions until I get one that is good for at least 150 league tokens
3) gift chest and sometimes fine gift chest from the gift shop if there is some particular girl that I'm trying to rank up affection; ignore this otherwise
4) action points, where the first two refills per day only cost 50 diamonds, and then next three 100 diamonds. Those costs reset daily, so I generally spend 400 diamonds per day for the extra 200 action points.
Occasionally an event will have something worth spending diamonds on. Spending diamonds on gachas is generally poor value unless either you're a whale and need more stuff to spend diamonds on, or you're trying to spend little time on the game so that you don't want the action points.