SkyBlock vs OneBlock
SkyBlock starts you on a tiny island with a tree and a chest; OneBlock starts you on a single block that regenerates and changes through phases (plains, underground, snow, ocean, Nether, End). Servers usually give each player their own island with upgrades and team invites.
First steps on SkyBlock
Chop the tree but keep the saplings and replant them, take the lava and ice/water from the chest, craft a pickaxe and build a cobblestone generator: let lava flow towards water with a gap so cobblestone forms in one spot (water must not flow onto the lava source, or you get obsidian). Expand with cobblestone while sneaking at edges. Then farms, mob towers and the server shop.
First steps on OneBlock
Mine the block under you, immediately build a 3x3 platform, keep saplings and seeds, loot chests before breaking them and gear up before hostile phases.
Choosing a server
Check void-death rules, the economy and donations (pay-to-win generators ruin leaderboards), wipe schedule and online count.