I'm a TA in my ds's school and started as a volunteer. I went in four days (chose not to do Mondays) and worked my bollocks off.
My aim was to make myself indispensable and show what I could do. I created lots of lovely displays, jazzed up book corners, and helped out with anything and everything.
I learnt so,so much while I was there that I never once resented the hours I gave. The skills I learnt, the jargon,behaviour management and just being known as reliable and able to do a good job-all totally invaluable.
I was offered two paid hours a day to cover sickness,that led to three months temporary full time contract. Which led to a permanent contract 
The headmistress was so impressed by me, she gave me a pay rise in three months, is paying for me to do my level 3 qualification online-I have to keep this secret as not all TAs get this. I am now first aid trained (extra money) and hope to do the HLTA qualification after.
I am a single parent and love that I can work,have no childcare, get to see my boys in the day and I bloody love my job.
I still work hard and often stay late. Help out everywhere. It's a job that is in demand and you have to show the school that you are what they need. Anyway you can. Quibbling about extra hours might mean someone else will be happy to step in and do those. Volunteers are plenty.
I wanted this job and went all out to get it. I smile every single day and I adore my class. I sing,dance and laugh-but my class know I mean business!
HTH