OK, I'm a teacher with about 20 years of Primary teaching (infant and junior) behind me - but I clearly remember being in your position. Don't panic. It will get easier as you get more experience and the fear of getting through this year is behind you.
Now, a couple of tips :)
Try to lower your voice when you ask for instructions to be carried out, and slow down. If you are stressed, or panicky that the children won't do as you say, the voice tends to rise in pitch and speed and this makes you less authorititive. It's a useful tip in sounding more in control without being scarier.
When you ask for something to be done, expect that it will be. Don't even let the thought "What am I going to do if child x doesn't obey me?" enter your head (it's hard, I know!) Work on controlling those inner negative thoughts. The very fact that you assume it will be done will communicate that assumption to the children - and they will be much more likely to do what you say.
If a child is refusing to do what you ask, use the broken record technique. Keep to the same low, steady tone of voice and ask again. Repeat. Repeat. The child will give in.
Speak quietly so the children have to listen to you. In a class of noise, it's tempting to raise your voice over the noise, but this tends to just make everyone noisier, and you will end up with a class who ignore you when you speak. Get in the habit of talking quietly so the children have to listen.
Smile a lot, especially with children this young. Praise them at every opportunity you can find, and ignore the negative as much as you can. If they are lining up and 29 children are rioting but one is standing still, don't try and pull the 29 into line, praise the one - very pointedly. Say "Wow, Jane is standing so nicely! You can lead the line today." Immediately 3 others will follow suit, say "and Freddy, and Jack too! Goodness me, we're doing well today!" Before you walk down the corridor, say "I know we're all going to be really quiet because I have the best class in the school." Smile, smile again. Remember that little rewards mean big things at this age. Tell them you are looking for someone specially well behaved to go at the front of the line, and the back. Say you are looking for the person sitting the straightest to give out the pencils. Say you have a special sticker to give out to someone you catch doing something really good. when you catch them, make a big deal of it.
They are so young at this age, you can get away with so much of this! They need lots of praise, lots of encouragement, lots of thinking that you like them better than any other children you've ever known. Primary school children of all ages will bend over backwards not to disappoint a teacher who they genuinely like and admire. Take a genuine interest in them. Edna is right that you need to pretend to really like them - and the rest will come.
The class is yours, not the TAs. Shouting at a reception class is not a sign of good teaching or good control. It's hard in this situation, I admit, but I'd try to direct her activities a bit more so she doesn't have quite so much control. It is tempting to lean on a TA who has better control than you (if only because they are scaring the class into submission) but this is counter-productive. You need to find your own way. Who is your mentor for this year? Can you discuss this issue?
Sorry if you know most of this stuff already and it is all basic rubbish! Hang on in there and go back with a smile. Hopefully by Easter you will be looking forward to your return!