You need to do it in progressive way. If you load the bar with any extra weight, than what your muscles are prepared for, that may cause injuries. I'd say go do a progression of 2.5 lbs for a couple of weeks. In a matter of 36 weeks you'd be able reach the goal of 45lbs which is 20 kgs. By that time your muscles would be stronger to handle the load. That's the safest way to do it. In this ego driven fitness industry, lifting heavy may matter, but staying safe matters more. After all, you don't want to hurt your shoulders or any other muscles and be out of training for months. Then start from ground zero. So, give yourself the time, and do it gradually. I'm not against heavy lifting, but I have seen people who bench 500 lbs, but they could barely play with their kids or do some daily normal activities without pain.
So follow the weekly approach and get ready for the heavy thing, and keep eating clean and give your muscles adequate rest time which is around 48-72 hours at the level you are, or which ever break suits you. Also, as Jj has suggested, going for a personal trainer would be ideal.