You won't build much credit in six months.
The way you build credit is slowly, through wise use of credit. People who don't use credit wisely pay unnecessary interest charges and get themselves in the hole where all they're paying every month is the minimum payment, which is designed to keep you in debt, keep you paying interest.
The way to build credit is to first, decide if that $200 - $300 item is something you NEED or something you WANT. If it's something you NEED, then buy it and the first billing statement you get, pas as much as you absolutely can, not just the minimum payment. The goal is to get it paid off as soon as possible. A $300 item can be paid off in two or three billing cycles. Get a second temporary job and you can have it gone the first month and start putting money away.
If it's something you WANT, you are already headed down the wrong road in life. Before you start buying what are essentially adult toys, there are other things you need. You need a savings plan. If you are covered by someone else's health insurance, there will be a time when you won't be. Having life insurance is a good idea. There might night be a spouse and children yet, but getting insurance at a young age insures your insurability. What if you have an accident that causes a life-long medical issue? You may not be able to get the insurance you want later. Now, you only buy on credit what you need. It's too tempting to whip out that card and pay for little this and that, impulse buys, the $6 Starbucks several times a week, meals, movie tickets, etc.
Even if you do everything perfectly, six months is not enough time to increase your credit score. Your score may even drop at first. If you run right out and put $300 on your $1,000, you've started out with a utilization rate of 30%. The scoring companies don't like that. Ideally, you want to keep your utilization rate below 10%. Whatever the total amount of your credit limits is, don't carry a balance of more than 10% of that, and if you do, get it below 10% ASAP.
The first thing you will see that will tell you you're on the right track is when the credit card company raises your credit limit. It might only go up $200 or $250 at a time, but it will go up if you use the card wisely and pay, every month, as much as you can. It took me a long time to get there, but I've been at the point, for over ten years, when I never carry a balance on my cards. I haven't paid a penny of interest in all that time. Whatever I buy, I have the money to pay for it and use the cards for convenience and easy record keeping. I pay the bill before the due date.