Tristram Runs in Diablo 2
Tristram Runs, also known as Tristy Runs are the most popular way for players to level up in Diablo 2 from level 1 to 15. A Tristram run consists of clearing every enemy in Tristram before reseting the game. You can find the town of Tristram by progressing through the third quest of Act 1.
In order to reach Tristram you'll first need to acquire the Scroll of Inifuss from the tree in Dark Wood, simply walk up to the tree and click it to retrieve the scroll. Show this scroll to Akara in town who sends you to the Cairn Stones in Stony Field. Press the stones in the same order that is shown on the Scroll of Inifuss to open up the portal to Tristram.
Once you complete this part of the quest the first time you won't have to activate the stones a second time as the portal will always be open for you.
The Tristram map is quite small and never changes. In the center of it is Deckard Cain, one of the most famous NPCs in the Diablo series and also part of the Act 1 quest that involves coming here. Typically you'll find two or three champ/boss packs in Tristram with one super unique that always spawns here, Griswold. If you've ever played the first Diablo you may recognize the name as he was the Blacksmith of Tristram.
Last but not least you'll also find the corpse of Wirt, another NPC from Diablo 1, lying in the northwestern corner of Tristram by the river. Click on Wirt's body to spawn a bunch of gold piles and an item called Wirt's Leg which will be either white or gray for you. Wirt's Leg is part of the quest that will grant you access to the secret cow level. For more information about the secret cow level follow the link provided to be taken to my guide all about that location.
As far as leveling goes, you'll want to keep doing Tristram runs until you are level 15 at which point you'll want to either switch to doing Tomb of Tal Rasha Runs or complete Act 1 and the majority of Act 2. If you're being power leveled then switching right to Tomb of Tal Rasha will be the best EXP but if you're soloing you won't be able to reach the Tomb of Tal Rasha right now let alone clear it.