Is she willing to sit down and read unbiased factual information on the steroids you plan on using that are backed by sound science? If so, share the facts with her. If she is so close minded that she refuses to even entertain the thought of supporting your steroid use then you have 3 choices:
1-drop her and find a girl who can accept your lifestyle
2-use your wit--find an angle and work it to win her over/convince her AAS=okay
3-lie and use anyway (this will probably lead to her constantly interrogating you, making accusations, going through your presonal shit looking for evidence).
4-let her irrational fears and lack of knowledge dictate your actions and don't use AAS
4-tell her you are taking them anyway and she'll either make your life hell (see #3) or eventually come around and realize its not a big deal
I woudn't recommend number 3. Number 2 is the best but you have to work a good angle. Number 1 is easy on paper but if you care about her it'll be hard.
I've had to deal with this a few times. Unfortunately, most of the girls I know have a negative--make that extremely negative view of steroids. I can tell you my experience with this issue and give you a couple of pointers, but I think the women here are probably going to be a better resource as far as how to deal with the anti-steroid girlfriend situation.
When I started juicing I didn't keep it a secret from the women I dated which looking back was a mistake. They would either find out because I told them early in the relationship or by "accidentally finding" (aka snooping around) evidence. It may have been due to the fact that it was early in the relationships, but most whom found out would get very upset and essentially give an ultimatum--them or the AAS. They felt steroids were rediculous and on the same level as hard drugs like meth, heroin, etc. Without exception, they refused to have a reasonable discussion and that was the end. After that I stopped telling girls who I was with about my steroid use, and most of the time I wasn't using so it wasn't hiding anything. But after a string of short/quickly terminated relationships I'd decided it was better to keep quiet, both for personal/sex life and for legal purposes (one of the ex-GFs devoloped a bit of hate for me and tired to pull some shady shit on me involving my AAS usage/possesion).
The two girls I've been with that didn't/don't mind BOTH found out the same way--they asked me a few months into our relationship. The first time it was shocking. We were fooling around on the couch while watching TV and she just stopped, sat upright and asked me point blank, "do you use steroids?". I was really sweating, man, I didn't know what to say and before I could confirm or deny it she started saying, "the reason I ask is because you are really big and muscular and I've been doing a lot of research on steroids lately because I've noticed that you get fluctuate in weight and get bloating that guys shouldn't have." While I was trying to come up with an excuse for that she then took off the gloves and said, "this isn't an insult but your balls are a lot smaller than normal which is supposedly another side-effect". At that point I was mad and my ego was hurt and I honestly was ready to break up and I told her the truth in GREAT DETAIL. She was suprisingly cool and we talked about it for awhile, and I answered a bunch of her questions. But the small nuts thing bothered me and made me feel ill-equipped so I told her that I could take some things to make my testicals bigger and she was very much against it--she confessed that she thought big nuts were "gross/ugly, get in the way" and some other shit I'd rather not repeat. Anyway, she was really cool and eventually did a cycle of clen and primo tabs before moving back to the east coast which is when we parted ways.
My current G/F asked me about a year after we started dating. We were talking about her breast implants and she just flat out asked me if I had ever used steroids. I was totally honest with her b/c I consider her to be very intelligent (despite the blonde hair/fake boobs). She also told me she was fairly certain I had at least done them in the past because, brace yourselves--my testicals were smaller than most guys who were otherwise normal sized. I gave her some shit, jokingly asked her ''how many dicks have you sucked" (clerks ref) and she just laughed and said, "I'm a nurse, asshole...about 50" LOL (she was kidding I think).
Anyway, she doesn't exactly love the fact that I take them but I always remind her that it is no different than her fake breasts. Especially when she's told me (when I'm totally out of shape like I was after my first hernia surgery and hadn't lifted for 4 montsh) that I'm in great shape and don't need to do them anymore--I always tell her I wouldn't mind if she had her fakies removed but I know she likes them as much as anyone else and its the same for me & AAS.
Sorry for the rambling, just sharing my small amount of experience, but basically you just gotta hide it and take hcg/clomid to keep the nuts normal if you plan on lying about it. But if she already knows you're interested and/or are going to use them be prepared for constant fights. If you think its going to get serious you'll have to come to some sort of agreement, IMO. Just explain to her that its not much different than her taking birth control pills, getting fake tits (if she don't have 'em buy her a pair and she'll forget about the roids, LOL) or wearing make-up. Basically you have to find an angle and work it pretty hard to convince her to be okay with it. Coming up with something shouldn't be too hard, I mean if you can talk a girl into sleeping with you then you should be able to talk her into letting you use steroids. Or maybe you don't have that kind of difficulty bedding women, hehe.. Honesty is pretty important in a relationship though and if you do lie about juicing and she finds out it won't be pretty--she won't trust you and the guilt trips will be unbearable.
Good luck.