Irishguy22 saidI've been going to my gym for months and for the past while I've been trying to increase the weights of the dumbells I use but I get three curls in and my arm wont raise after that. Im not a strong guy im only lifting 12kg, and the next set up in my gym is 14kg. Does anyone have any suggestions on how I can improve?
You just need to reduce the weight and slowly build your strength up. First of all, I'd recommend picking up a copy of this book as a basic introduction to weight lifting & building muscle: http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp/0982522738/ref=sr_1_1?ie=UTF8&qid=1409244642&sr=8-1&keywords=starting+strength
Second, you need to start with a weight where you can do three or four sets of 8 reps comfortably. Then continue on until eventually you can do 12 reps comfortably. Once you reach 12 reps, increase the weight slightly and drop back to 8 reps. Build up your strength to 12 reps. Then increase the weight, drop back to 8, build up to 12, rinse and repeat.
One of the most common mistakes new lifters make is trying to jump straight away into a weight they cannot lift. So they end up either 1) discouraged and give up, 2) using poor form and possibly getting injured, or 3) making no progress ever. Give up on the idea that you are "weak" and that there's something wrong with you for not being able to lift what bigger guys can lift. It's not that you are weak. You are just new to this and need to build your strength, and with proper patience and training you will quickly increase. I remember just a few years ago I couldn't bench press 100 lbs. Well look at me now. You need to start with the basics and work your way up. Do not be in a rush or you will find yourself sorely disappointed.