1. Write some songs with the band, and make sure that everyone's on the same page about what kind of music you're playing. If your best friend thinks you're playing pop, your sister thinks you're playing punk, and you think you're playing pop, there are going to be some problems.
2. Like other people said, get a drummer and a bassist.
3. This one's important. Be ready to fight with your sister and best friend. Fights are going to happen. They're inevitable. It's hard to be best friends with someone in your band. I promise you will fight, and you will hate her, and you will want her dead. Everybody says 'it's not going to be like that for us, we're stronger than that, we're different.' You're not. You're all human. I can't stress this enough. You are going to hate everyone. Be prepared for this, and learn how to let things blow over.
4. Make sure everyone is dedicated to the band. I think someone said this before. If your best friend thinks this is her future, and your sister thinks this is a way to attract boys for fifteen minutes, things are not going to work out.
5. Become your band. Embody it. You have a name, great. Be it. Live it. Know what your image is, and be it. Take some pictures, draw some album covers, hand out fliers, call each other by stage names. Even if you don't use any of it at all, it'll still get everyone in the groove of things.
6. Get people excited about your band. Not just peers, but family, strangers, everyone you come in contact with. You'll have support everywhere. People know people who can hook you up with things. Did you know that Uncle Greg has a Les Paul that he doesn't use anymore? You would have never know that if you didn't tell him you were starting a band.
Also, if you let the idea die down, people will start asking about it. They'll say, 'whatever happened to that band you were starting?'. Maybe you can resurrect the idea.
7. Practice whenever you can, but don't do it so much that you hate playing. If you're serious about the band, if this is your thing, you don't want to have fantasies about blowing off your head every time you play a certain song.
8. Along with writing songs, you'll want to get some covers down. I heard once that a good band has at least 100 covers to pull out at any given time. A good cover is one that many people recognize, but one that you can put your own twist on. A good one, but very over used, is Smells Like Teen Spirit by Nirvana.
9. Listen to a lot of music. One thing that I've found helpful is singing along to music, but adding my own harmonies. Even if they sound bad, they might inspire other ideas.
10. I'm not sure how you're learning the guitar, but I would definitely recommend learning chords. They'll give you confidence (you can finally sing and play a guitar at the same time!), and it'll give you a musical ear. From chords, you can play around with notes and keys, and it'll open a whole new range of opportunities.
11. Get an ego, act like you've already made it, be arrogant. But be prepared to be rejected. Have confidence. Be prepared to have a lot of things said about you. They'll say 'you're not pretty enough', 'you're not skinny enough', 'you're not good enough'. They'll insult you in ways you never even thought possible. You'll be rejected more than you'll be accepted, and not just by fans. Record labels are insane. It's ridiculous, but unless you have enough money to start your own record label, it's something you have to go through.
12. Things will never go as you plan. Like, my first gig with my first band was at a church. In the bulletin, we were labeled as a 'heavy metal Christian girl band'. None of us were Christian, we weren't heavy metal, and our male members weren't very impressed. You just have to deal with it. Accept losses and don't celebrate too much. Take things as they come.
13. Be completely honest with each other. Don't be afraid to hurt feelings. If your band's purpose is music, don't make the purpose friendship. Your band is a band. If you hear something off, tell someone about it so they can fix it. And tell people to do the same for you. You'll never get any better if everyone just says 'That's fine, ok, whatever'. Be the best. Work hard.
14. Set up boundaries, talk about morals. You'll always be tempted to do something you told yourself you would never do. For example, someone who has been sheltered all their life will not be the same after going to a few parties. Discuss and understand that new things are going to be happening, and changes will always be coming.
Hope this helped.