First you need a email so you can get steam then you go to the steam store and buy terraria

To make a new world once you have Terraria you enter the game then you press multi or single player and press new player then you can create your character after you have done that select your character and press new world do your assets once your done that press create world and then same thing enter it

Now your in your world so you should have a copper/iron shortsword equip it and wait for the slime to spawn. Once the slime has spawned it is time to kill it, with the shortword equipped you can press the left mouse button to swing your sword out. Now this is a shortword so you will need to risk getting close to the slime and taking damage. It is a fierce battle but with a few attempts you should be able to kill a slime.

If you are doing a run were you don't use melee weapons such as the shortswords at the start. You can craft a wooden bow by chopping down trees to collect wood and make a workbench, then chop MORE trees down to collect enough wood to craft a wooden bow, then go to the nearest caves, break some pots to find arrows, and then shoot the slimes.

If you don't know how to cut a tree, please refer to my other guide here: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3126361981

If you are doing a mage run so you can't use the shortword or bow then I suggest looking for a wand of sparking in surface chest!

If you are doing a summoner run then you COULD look for the finch staff (or alternatively get it from the start of a journey world) but it is very rare chance to spawn in living wood trees. I'm sorry but i don't really know any better summon weapons you can get before killing that damn slime, so I guess you can either waste precious time looking for the finch staff OR give up on summoner.