Welcome back! Start with quests. A lot of quests give great exp rewards that will get you decent base stats. Additionally quests will give content access to monsters thing and OSRS gold slayer masters to you. After quests, continue with slayer. Personally when I came back I started out doing quests by looking about the school wiki at them in the sequence which they had been published. I strongly suggest watching Slayermusiq1's guides on YouTube, if you do not care about the story line, he helps get them through fast by telling you that dialogue alternatives to choose.
No problem! Would recommend you start with Waterfall pursuit, it is super easy and will provide you some wonderful strength and attack exp. Beyond that, there's a few rather easy. Town, dwarf cannon nature spirit, fairytale grand shrub, part 1 and tree gnome village. Also complete all of the f2p quests as they're very easy and fast, they will offer you some speedy exp and pursuit points.
Really though, you can not go wrong regardless of what quest you do, then you will have to do them all someday as they're all a necessity for something. Do the quests that give farming experience to you and start doing herb runs to passively make some cash. Doing Recipe for Disaster pursuit for accessibility to Barrows Gloves is a necessity for every account that is regular, and working towards it is going to help you train your skills all.
I'd just keep on with slayer, divided by as many quests as you can stand to do lol. Otherwise, I would recommend going for 50 in most skills and buy RuneScape gold knocking out the easy and/or moderate achievement diaries. Trust me it's far better to get those done early to begin reaping the benefits whenever possible. Also huge note, don't purchase bonds with gp. It'll keep you broke for a long while that is fantastic. You start accumulating you won't have to use welfare equipment when you start bossing. That is most new players largest mistake, and why you see a few lvl 100s with 50m+ bank and a few that can't manage a bond.