Not at all. You are the same height and weight as me. I've never worked out ever before, and plan to start this week. But I know I can do 20's.

Should I also add that I'm 15. I dont think that counts towards much would it?

It doesn't. 6kg is nothing. You arn't trying hard enough if you can't lift more.

Cool, I'll see how much more I can take tomorrow. Is it more important to do bigger ones with less reps or smaller ones with more reps? I'm thinking that more reps would just add endurance.

Bigger ones with less reps if you want muscle. The other way around if u want endurance.

Thanks.

Just because I'm a nosy arsehole, How old are you?

17. I'm just as weak as I was when I was 15 :p

Another question, is it normal for your muscles to be tight after a long workout? I know the day after they should be sore but right after the workout should they be feel tight?

Yes, it's normal. Also, ignore everything catzombie said (he admitted to never having worked out before).

The optimal rep-range for growth is between 6 and 12. As for the weight, it's really up to you; pick a weight with which you can't do more than 12 reps. As you continue lifting you'll be able to use heavier weights; starting heavy just for the sake of lifting heavy shit is a really bad idea.

Only you can tell exactly how well you're doing. If you can do 6kg for 13 reps with ease pick a heavier weight. If you can't do more than 6 reps go with a lighter weight.

P.S. I'm talking about slow and controlled reps, using momentum is a big mistake many people make.

Just for future reference:
Strength: 2-5 reps
Volume: 6-12 reps
Endurance 15+ reps
This is not the case with everyone, but it's accurate enough for most people.