first time bleaching?

I'd say you would have better luck buying bleach powder and developer seperately than using a kit. When I first bleached my hair, I posted on this forum and that's what everyone suggested. If you buy a kit it can sometimes be more damaging or be too strong/weak and could cause uneven results. You can buy some at your local Sally's or other hairdressing supplier depending on where you live.

Also, before bleaching you should apply a generous amount of coconut oil on your hair and add the bleach on top. This will protect your hair and limit the amount of damage done.

So before you apply any bleach you generally need to backcomb the hair before to get a nice graduation rather than a harsh line. Some people may use one strength of developer and apply it at the bottom wait a little while, then apply bleach on the midlengths, wait and then finally do the top. Other people may use different strength and use the highest strength on the ends, a weaker strength on the midlengths and the weekest strength at the top.

I'm pretty sure there was a detailed post on this forum explaining how to do an ombré.

Sorry that was quite vague. Hopefully someone else can give you better advice about what kind of strength developer to use and how to apply it.

The technique described above would give you an ombre (dark gradually getting lighter towards the ends) but I don't think that pic is actually an ombre, it looks more like balayage to me, The ends are bleached solidly, then the bleach is hand painted vertically to create highlights to blend the solid area in. The bleached parts at the top of her head look just as light as her ends to me.
I would guess that the girl in the pic had it all bleached and as the colour has grown out she has had the balayage effect added to soften the regrowth and create the rooty look that is so popular atm.

Wicked Pixie, yeah I think you're right actually. I didn't actually look at the photo close up (I'm on a phone). If you'd prefer the look of a balayage ombré rather than a normal one, I'm sure if you go on YouTube I'm sure there are plenary of balayage ombré tutorials. I know Guy Tang does quite an in depth video on how to do one, which would just show you the kind of placement to put the bleach. Whether you like the guy or not (lol that wasn't intentional), I find his balayage ombré tutorial (he has a few but he does one longer one) pretty clear and easy to understand how to do it. If you'd find it quite complex to do on yourself you could always get a friend or someone to help.