Stretch your muscles before and anfter you exercise. When you exercise; your muscles tend to go tense.
Don't worry about the loose muscles bit, it's great that you are starting low and working your way up, however; you should focus on adaptation.
To adapt to something like the weights workout, you should start at a low weight, stay at this weight for a few weeks (to allow your body to adapt to it and then they build themselves up to deal witht the presure and intensity of the work load they have to lift) and them alternatively work your way up to the next one.
Don't throw yourself into the next load up, because you're going to hurt yourself. What I suggest there is you stagger the work load (say...5 kg, the day after 8 kg, back to 5 kg, then back to 10 kg so the muscles get used to it).
Also core strength is a major. Core strength goes with absolutely everything, that is the thing that holds you up, work on that too if you want to be able to lift statically.
Don't even try to think you can't go any further, you can, you just need the mindset for it. Think positively over it, it works, especially when you exercise you need to be patient and you need to be very considerate about how you go with exercising. A positive mindset is what works most.