Okay so yeah you want to start off with just the weight that you can handle for now. But each week try to increase the weight just a little bit until you can get up into the heavy weights.
While you're still lifting lighter weights go for more sets and higher reps. For example 5 sets of 15 reps. Once you're able to handle heavier weight go for like 3 sets of 6-8 reps.
As far as your diet, since you're gaining you'll need to add more calories than you normally consume. Definitely track your calories and try to raise your intake each week or biweekly depending on what you can handle. You'll see quick results consuming more calories and lifting heavy weights.