Dude, fair play to you for asking. I'm new to using steroids as well, but have a degree in Sports Science and been lifting for 8 years. Common sense goes a long way and educating yourself on forums like this one is a great idea as it's full of people who know what they are talking about. If someone hands you a half used bottle, don't trust it. 1) It could be anything. 2) He could have re-used needles 3) Half a bottle won't do much (if anything).
First things first. Ask yourself what you want to achieve. Then research into what diet you need to help reach that goal. Nutrition and diet is key to gaining muscle, there's a lot of stuff on the internet about what foods you should/shouldn't eat so I won't bore you. As for your workout, get a plan sorted out. Again, there's loads of stuff on the internet that you can use if you don't know how to plan a regime yourself. Also you need commitment, it's not going to happen over night, it's a long, painful, tiring but ultimately, enjoyable experience. I trained for 8 years solid, completely cleanly and I like to think that it's shaped the person I am today. I didn't take any shortcuts and I worked hard to get my physique. Only now that I've levelled off and it looks like I've reached my natural limits am I considering taking steroids. I recommend you do the same, it's hard work and a long process but you'll be glad you did it all yourself. That's not to say that taking steroids earlier is wrong, it's just how I feel. At the very least, get yourself a solid base before taking anything. If you have a solid foundation of muscle before using AAS you'll be all the better for it.
If you want a sample gym regime or diet advice feel free to PM me.
Good luck.