If you think it's tough for you to build muscle because of your body type, using steroids won't help. Instead, focus on eating more, working out harder, getting enough sleep, and understanding how your body functions. Soon, you'll realize that you're not a 'hard gainer.' Once you've grasped these basics of muscle building naturally, steroids might be more effective.
For your query, starting with a daily intake of 2.5mg Anavar for 8-16 weeks could be a good beginning. If your blood results are good, you could consider increasing to 5mg after 8 weeks. Combined with following those fundamentals well, this approach has consistently shown progress for my clients. Keeping everything in check is key for a simple cycle like this to work effectively.