Which type of model to use and what's my target variable I am a beginner in Data Science field, so sorry if my question is too basic. 

The task is to build an ad bidding model for online marketing which allows you to deliver targeted ads to the right people.
A part of the given data is 


I don't have any additional info about the task. Is my target variable the variable spent ?
And if yes which is the best method to follow to predict the target variable?
I would say the variable 'spent' is your target variable. I'd suggest you to analyse the distribution of each feature and target to check if, eventually a linear regression could be appropriate.
You might try different other methods however, like a decision tree regression, which does not require any assumptions as it doesn't estimate parameters.
What I would suggest you to do, howeveer, is to concentrate on the distributions of parameters to check for eventual unbalances and make groups to make estimation robust.