There are many paths to choose from at any point in the game. I'd say it's best to just figure out your options and choose whichever one is the easiest at the moment. There are white sign soapstones that you can place and then you'll summoned by someone. Your goal is to follow and protect them and help them achieve whatever it is they need help with. The final goal, however, is beating the area boss which will them return you to your own world. This will not defeat the area boss in your world, only theirs. You can use an Ember to become "cindered", which will then allow you to see other players' summon signs, giving you the option to summon them to your world to help you with whatever you need. Anytime you're cindered, other players can invade your world and attempt to kill you. You can not control whether or not they join asidr from not being cindered. You become uncindered when you die. You can also invade others' worlds by using a Cracked Red Eye Orb or a Full Red Eye Orb. Cracked is single-use. Full is unlimited use. These items must be found in your world. There are also Covenants. You get different items by participating within these covenants. While in some covenants, you'll automatically invade other players. Some covenants have special things about them. You can look that up yourself if interested.