At my school the teachers are running exam technique sessions at lunch times for history to help you with answering the questions correctly. You could ask your teacher if they could do something similar for you, or you could answer some practise questions at home to time and ask your teacher to go through your answers with you so you can see where you're going wrong. They should be happy that you are so motivated to improve your grades. Then focus on your weakest areas in the run up to exams until you're confident with the technique.
For sources remember to look at the source as a whole as well as the smaller details. Use the 5Ws to help you to think about all aspects of the provenance- who made it, when was it made, what is it (poster/ report etc), why was it made (creator's intentions). It may be helpful to make a (brief) plan for source questions on usefulness- content useful/ not useful, provenance useful/ not useful, and then make your overall conclusion.
As for using irrelevant information in questions, try linking events together on spider diagrams so that you can see how different causes/ effects are connected. Create timelines of events so you can see the chronological progression of the topic you are studying and what changes occur (structuring your answers with this in mind may help you to gain higher marks). Always read the question carefully to make sure that you don't talk about any events that happened outside the years that the question is asking about. Good luck!