Last year was my first launching year so I was quite concerned so I did like you and asked the good people of ribnet
The first site I used was hamble point, great but expensive. To be honest the number one advantage was that there was no onlookers so I coudl spend and age gettng it right (or as right as I could)
I then used the slipway at Swanage (not the town one, but the one by the lifeboat).
Very wide, you can park your trailer and the car park is next door.
It also have an attendant during the day.
Swanage gives you plenty of fun going to the left.... towards poole...
To the right is quite wavey water but again well worth a play
Landlocked in Northampton