Taking it on a Friday is a decent plan, it takes between 8 and 72 hours to reach a peak in your system, so that should be over the weekend. You likely won't get side effects immediately, or mild ones if you do.
When you get the package, check that you've got the medication - it comes in a cardboard box with just the injection pen inside. Check it's the 2.5mg dose and is in date according to the expiry date printed on the pen - it will be, but I always like to double check. Then read the two paper leaflets that come with it. One is about the medication itself, the other is about how to use the pen. Nearly all your questions will be answered by those.
Also check that you have the separate needles, they are quite small and easily missed as they are not in the cardboard box with the pen. Most pharmacies also supply alcohol wipes, needed to clean the end of the injection pen before attaching a needle. If the pharmacy didn't supply any you can buy some online eg from Amazon. They should also have sent you a sharps box. If they didn't, you can also order one from Amazon. Small ones are only a few pounds.
When you decide to inject, get everything you need laid out, and follow the instructions in the paper leaflet step by step. If you are unsure about a step, you can always ask a question on here. I always inject into my stomach, lots of flab to choose from and easy to reach. Some people prefer using their thighs, you can also inject into your upper arm but given where you are supposed to inject it can mean getting another person to do it for you. The paper leaflet has diagrams showing you where you can inject. The needles are so short (4mm) that you are in no danger of hitting anything other than subcutaneous fat.
You don't need to worry about extra needles at the moment, the pharmacy should send you at least 4. The leaflets will explain about storage, but once you have started using a pen, it can be stored at room temp or in the fridge, the choice is yours. But you then have 30 days to use it, which should be plenty of time for the 4 injections even if you have to delay one. Unused pens must be kept in the fridge.