Firstly, welcome
Secondly, yes you can pay on the gate. £20 per adult, if you are in the ground before 1:00pm entry is £10. Concessions, £15 (£10 before 1:00pm). Juniors £5 (Free with a paying adult)
Parking is available on the club car park outside, usually £2:00 per car.
The popular side (standing) is uncovered - on your left as you enter the ground. Also the Kells End (standing), the covered end as you walk in. They are the most occupied. The Stand is seated and covered. The Railway end (standing) is also uncovered (where the electronic scoreboard is located). Currently weather is for light rain/sunshine over the duration of the match, so you may need to bring a waterproof coat.
The bar will be open so you are able to get refreshments and a friendly welcome and a crack with other Haven Fans
Hope that helps you