What Centre will you be bowling at? If it has a Pro Shop speak to the guy there as they would be able to help you with a ball selection. At this early stage it is very difficult as you don't know what style and weight you will be bowling. They may also be able to give you some basic instruction on how to bowl the ball.
Some centres also have coaches available who can assist a beginner bowler to find a style to suit you and that you will feel comfortable with.
As you have not bowled before see if you can get to the bowl a bit early and ask to speak to the league secretary or an experienced bowler who can go through some of the basic rules of bowling ie giving way to bowlers on the approaches, going over the foul line etc. This will avoid you having problems when it is your turn to bowl in the team.
Personally I would start with a house ball (take note of the weight you use)for the first couple of weeks at least to see how you handle the weight. It may be that you find the first weeks ball is too light or too heavy. Dont be afraid to change your ball weight until you find one you feel comfortable with. You can change your ball any time during a game. After a couple of weeks (especially if you can get some practice in during the week) you may find you need to go to a bit heavier ball as your arm strength increases.
Dont worry about your average. Every one is different. As straightkut said just do you best and try to improve a pin or two each week. It is much better to try to get a consistent shot than spraying a ball all over the place trying to go faster to knock them all down.
If you can improve your shot by just one pin a game each week then by the end of this year you will have improved by over 20 pins.
ps some leagues can be a bit funny with beginner bowlers, some are more casual, some are very serious, if you are unhappy with your league just go to the bowl on another night, look at other leagues but be assured there is one there that will welcome you with open arms and that you will feel comfortable in.
Enjoy yourself.... let the ball roll.. let the pins fly and welcome to the wonderful world of bowling.