Hi lynne
As the above said try and time it with the tide if you want to go down to the mouth of the Onk and play in the surf ,I have found just as the tide floods in at the mouth it will carry you all the way back up to the car park at the main road bridge. If you want something more sedate anywhere on the river from the bridge over to the surf club upriver is quite easy to kayak in as the river current is not too strong but you might get stuck in shallow bits so just get out and walk
From experience paddling in the wind is a pain in the bottom so try and avoid it, as it catches your paddle and dripping water blows in your eyes. Lots of great birdlife on the Onk if you take a cheap set of binoculars it's good fun, just give the swans time to get out of your way and don't crowd them. I use willyweather app on my I phone to tell me the tides times its really easy to use, there is no best time to go kayaking just go at different times that's the fun of being out in a kayak you can go anywhere anytime ( weather permitting) make sure you wear a life jacket, sun cream, hat, water and whistle all basic safety gear but I've used it all except a whistle thankfully.
If you have any other questions give me a shout, have fun kayaking.