I played Act 1 a lot due to EA, and I always go this way:

1. Recruit Shadowheart, then Astarion, then Gale (I always succeeded till now to get him, luckily also in my Honour run).
2. Ruins, get Lazarus.
3. Recruit Lae'zel, she stays in the camp.
4. Fight at the Grove. Level 3 after it.
5. I don't go against Kagha, I agree to help the refugees to get out there.
5. Recruit Wyll, he stays at camp.
6. Blighted Village, save the gnom, the hidden cellar (I don't do the spiders now). After that usually you are Lvl 4.
7. Go over to the gnolls, kill the gnolls at the street, clear the cave. I usually have Lae'zel here instead of Astarion.
8. Tollhouse, get all the hidden stuff. Look for Karlach, recruit her (she is the strongest companion but I usually don't use her). Clear the Tollhouse.
9. Over to the Village again, then clear the way to the Goblin camp.
10. In the camp get entrance from above though the wall, don't fight.
11. Go to the swamp, kill the dwarfs and Ethel, save Mayrina.
12. Go to the coast, kill the wandering trees.
13. Fast travel to the Goblin camp, go into the temple, buy/sell stuff the last time from the small trader, then
a) kill the Goblins where the prisoner is, free Volo.
b) kill the goblins with Priestess Gut (I usually lure her into her room, then kill the goblins, then her)
14. Seek the way to the Underdark, kill the big guardian, avoid the stupid sun circle enigma by searching the hidden lever to open, get the fast travel point in the underdark)
15. Back in the temple I
a) kill Dzor Ragzlin
b) kill Minthara
c) kill the rest of the goblins and free Halsin
15a. Fastravel to the Blighted Village, run to the Goblin Camp exterior and attack from beyond the bridge, let them come to you, it's a very easy fight. Perhaps do the spiders.
16. Usually you are Lvl 5 now, go to Waukeen's Rest.
17. Kill the Gith patrol, relatively easy with lvl 5.
18. Into the Underdark, I jump down on the right side of the entrance building.
19. Kill the Minotaurs.
20. Go to the mushroom village, kill the Bulette on first appearance (possible with lvl 5).
21. Clear the village on the shore from the dwarfs, thereafter kill Glut
22. Clear the Hook Horrors
23. Clear the Magical Tower, defeat Bernard.
24. Roam around to do the rest in the first Underdark section.
25. Use the boat to sail to the Underdark Fortress.