Question about using tampons? Please help!!!?

I'm 11 and this is the fourth time I have gotten my period. Tomorrow at school we have gym cl and we have to change into different clothes so I thought I should wear a tampon. So I put one in today to see what it's like and it will only go in half way no matter what I do! I'm totally relaxed and not scared so I don't know what's wrong. I try wriggling it and pushing but it's no use! And I'm really tired of using the silly big bulky gross pads so I'm really upset now. So how can I get it in? And also, at night is it okay to sleep with a tampon in?