What exactly are you nervous about?
Not professionally it won't be anything, I'll put it this way in terms of damage: 1-10 in damage, 1 best 10 dipping your head in acid:
Using shampoo- 2
Using a hairdryer- 4
Using a flat iron- 7
The thing about bleaching is you do it, at most, once every 2 months, and it isnt too much more damage than most people do to their hair daily. Hair can be bleached twice a month and be fine, its when you bleach it, wash it, blow dry it, and iron it every morning that it starts becoming damaged.
Microscopically bleaching puts little nicks and dents in your hair, when you put dye on it the dye fills in the little nicks and dents, so after dyeing its going to be about the same as it was before.
The mistake allot of people make when bleaching their own hair is that they don't mix it properly. To make hair bleach you get a bleaching powder and a liquid developer and mix em, you have to make sure its mixed good or little chunks of bleach powder will burn your hair (scale of 1-10, 10).
As long as you don't use clorox bleach or some shit, you will be fine. Bleaching it in moderation is perfectly healthy. If you're worried about your hair, just get shampoo that repairs damaged hair (but is also gentle on colored hair) and you should be good to go.
Good luck with the dying :DDDDDDDDDDDD
If you ever wanna get into doing your own hair (and trust me, you do) heres some simple instructions:
Get a bleach powder and a liquid developer (I have a big ass 32 oz bucket of Clairol Professional BW2 bleach that costs $6 and 32 oz Salon Care 40 Volume Cream developer that cost around $5 from Sallys Beauty Supply).
The bleaching powder should come with a little scooper thing, if it doesnt use a spoon, size doesnt matter, just use 3 parts developer to 1 part bleach (so for every scoop of bleach powder, put in 3 scoops of developer, this is the best ratio to avoid damage).
Mix it in a glass cup, plastic cups tend to not wanna wash out, unless you plan to throw whatever your mixing it in away. Make sure you mix it enough so its a smooth cream, it should look like you made a big donation at the sperm bank and not a cup of tapioca pudding.
Put it on your head. For bleaching, your hair doesnt have to be clean or anything. Let it sit until your hair is to however light you need it to be. If you plan on dyeing it yellow or something it has to be almost platinum blonde, if your dyeing it dark red or something just get it slightly lighter than the shade of what you want. The instructions say to stand under a blow dryer but thats not necessary, I've never bothered at least.
Wash it out in the shower, don't dip your head in sink, it has to be mildly thorough. Theres allot of myths about people going blind because they got bleach in their eyes in the shower, but thats bullshit, I've bleached my hair, my siblings hair, my moms hair, my dads hair, my aunts hair, my uncles hair, my friends hair, fuck I've bleached so much hair I'm ready to do anuses and have yet to see anyone go blind!
Your hair should now be bleached. If it isnt do it over again with more developer than the last time.
Coloring/Colouring, whatever, dyeing hair-
Wash your hair with shampoo, don't use conditioner, it'll keep the dye from sticking. Preferably use a shampoo that says "anti-residual" on the bottle.
Dry your hair. Dye will stick better if you use a hairdryer prior to dyeing (the heat opens the hair follicles, allowing more dye to sink into the hair, rather than sit on top).
Some dyes say you should put it in a bowl prior to application, but thats stupid. Put on gloves (you'll thank me later) and squeeze dye into your hands. Or if its a bowl scoop it out. I don't feel this section was necessary.
Let it sit until it pretty much dries on your head. For the first minute or so blow dry it.
Wash it out, you can dip your head in the sink you must. Wash it in cold water, it will make your hair follicles close with the dye inside. The color will last longer if you wash your hair with cooler water from now on. Some people say to wash with a "hair color safe shampoo" but I've never seen any difference, and that shits expensive!
The people at the salon will lie through their teeth telling you "your hairs all damaged and stuff, buy this and this and this and this and it'll be fixed," they make a commission of whatever they sell you. Really fried hair will feel like straw (no, thats not an exaggeration. Like. Fucking. Straw.) and mildly damaged hair will sink if put in a bowl of water, although thats not exactly a test created in a lab by guys wearing white coats with goggles holding clipboards, so its none too accurate. Only way to undo damage is putting extra virgin olive oil in your hair for about 5 minutes and using a leave-in conditioner over night or whenever you sleep. Only leave-in conditioner I can recommend is Garnier Leave In Conditioner, mainly because its the only one that doesnt cost oodles of money.
If you read all that then congratulations, you now know more than most professionals.
Things I feel you missed:
You can get a mixing bowl (made for bleach/dye) and an applicator brush for under a buck at most beauty supply stores. Do not be fooled, bleach WILL damage your hair. It completely depends on how YOUR own hair reacts to the bleach. For some people, like steve, it barely has any effect. For others, their hair can turn to straw or even break off completely. I'm truely not trying to scare anyone out of bleaching their hair! I do it often, but Iwarn those who do to heir on the side of caution.
There are different volumes of bleach developer. 40 volume will do the most damage on your hair but will give you the lightest results. 20 volume will do the least damage but will give you darkest results (in terms of bleaching, it might leave you with a more orange shade then the 40 volume). 30 volume is, obviously, somewhere in between. You should wash your hair the day before you bleach, and not the day of. The natural oils from your scalp will help slightly protect your hair from the damage of the bleach. ALWAYS shampoo after bleaching and wash in warm water to completely remove all the bleach from your hair. I would advise against using a blow dryer while bleaching, you don't exactly want your hair folicals open while bleaching unlike when you are applying color. Bleach your roots last, They turn blond the fastest so if you apply to your roots first youll often end up with lighter roots and darker ends. Make sure each strand of hair is well coated in bleach to achieve an even color, the best way to do this is by applying with foil and a brush (though you usually need a friend for this and it can take a very long time if you have a lot of hair!).
A hairdresser that tells you hair is damaged isn't always lying to you, your hair probably is damaged considering most peoples every day styling techniques cause damage (straightening, blow drying, harsh products, and styling). This doesn't mean you have to spend all your money on their outrageously priced products..just follow these simple hair care tips: