It can be very tempting to overdo the weight on singles, but as a new lifter, the fun thing is you don't even need to approach anywhere close to your one rep max in order to reap the CNS (central nervous system) gains that'll come from it. If you're attempting singles for the first time on the push press (which should be done out of a power rack) just pick the weight you're doing for the 5x5 and add 10-15%. It should feel ludicrously easy, but the point is focusing on your speed and technique. It'll teach you to push heavy weights fast, and with good form. It's a huge morale booster, and my clients love it.
As your firm up your technique, and your CNS adapts to pushing heavy weights with technique, you can begin to scale up the weight in a similar progression to your 5x5. That said, as a new lifter singles shouldn't make up too much of your diet. They are great barometers of progress, and you won't get very fatigued from them because they aren't true maxes, but they simply aren't as useful as increasing volume over time to build muscle mass. When your newbie gains slow down, and improving CNS efficiency becomes more important to gain strength, then singles become more important.
As your firm up your technique, and your CNS adapts to pushing heavy weights with technique, you can begin to scale up the weight in a similar progression to your 5x5. That said, as a new lifter singles shouldn't make up too much of your diet. They are great barometers of progress, and you won't get very fatigued from them because they aren't true maxes, but they simply aren't as useful as increasing volume over time to build muscle mass. When your newbie gains slow down, and improving CNS efficiency becomes more important to gain strength, then singles become more important.