Simple Answer

Use a weight where you can perform between 5-10 repetitions for multiple sets , with great technique and where the last 2 repetitions are difficult , but not by any means to failure.

All Beginners Differ

No two beginners are the same, making this question very difficult to answer. Let me give you two examples of beginners in the weight room and show you why they would need to use very different weights. Imagine an 16 year old, very athletic rugby player who is just getting started with resistance training. He will likely need to learn the bench press in his first session and will see dramatic increases in his strength over the next few months. His starting weight may be between 30-40kg depending on his body weight. Now imagine a 65 year old male who is recently retired from knowledge work and wants to get into good health. This individual may needs months of resistance training before introducing the bench press is a good idea. The starting weight for this individual would be a 5kg DB Bench Press.

No Specific Weight

This is why I cannot provide a specific weight, though I can say that starting with the barbell and seeing how it feels is the best idea. For many individuals, the barbell will be heavy and they may need to spend a couple of weeks or months at this weight before increasing the weight.

Should You Be Using An Olympic Barbell / Doing The Bench Press

There are also plenty of individuals, especially females, who should not be using an Olympic barbell until they have developed enough strength to do so. Olympic barbells weigh 15kg for females and 20kg for males, so if your not comfortable with 7.5kg and 10kg DB bench press respectively then you likely shouldn't be using an olympic barbell. This is perfectly normal, starting with very light dumbbells is a great option for most people to build a base layer of strength before starting the bench press.

Get Your Reps In

When starting a new exercise, like the bench press, you will want to oscillate between sets of 10 and sets of 5 for your first few months of training. These sets should use the difficulty scale of RPE (rate of perceived exhaustion). This is how you judge the difficulty of the set out of a scale of 10, 1 being extremely easy and 10 being a max. When performing an effective set of between 5-10 repetitions we want the RPE to be between 7-9, again it can oscillate between the two.

Once you know an exercise well and have some experience with it, have done some difficult sessions using it, then consider adding in sets of 3s, doubles and singles. These repetition ranges should be avoided by beginners but will be crucial to long term progress.

Walking Through Your First Session

So you’ve arrived at the gym and are prepping to use the bench press. You perform a set of 10 on the barbell and it is pretty difficult, to the point where you think you could only do 1-2 more repetitions before your technique breaks down. Then you perform 4 more sets just like this one and on the last set you only manage to perform 8 repetitions. This is a perfectly executed first bench press session, next week you repeat the same thing with the goal of performing 10 on each set.

In another scenario, 10 repetitions on the barbell might feel like a warm up and like you could easily do 10 more. In this case, you need to keep adding weight until you feel like the set of 10 was challenging, especially on the last two repetitions. It is all about finding the sweet spot for you, where we create adaptation through difficulty but whilst not hitting failure.