Dtjytyk

I recently "upgraded" to D&D 5th edition. I decided to play a warlock. However, now I would like to be proficient in Stealth, and a warlock can not have this proficiency.

I am Warlock level 2. Is it possible to now multiclass into rogue, take just one rogue level, and then continue advancing as a warlock afterwards?

My stats are as follows: Str 11, Dex 18, Con 13, Int 11, Wis 6, Cha 18.

Yes, you can multiclass and take one Rogue level

You can multi-class just to gain 1 level of Rogue since you meet the requirements. As noted in the basic Multiclassing rules, you need a Dexterity of 13 and Charisma of 13 in order to multi-class from Warlock into a Rogue.

Alternative

Have you looked at backgrounds?

This is normally something done at character creation, but you could have a word with your DM to see if he is amenable to changing your character.

Each background tends to give proficiency in two skills (as well as tools and/or language). If all you really want is proficiency in Stealth, then you could find a background that grants this such as the Criminal background. If you can't find a background that you like you could also work with your DM to make a custom background (see PHB p. 124-126) that fits your character and grants proficiency in Stealth.

The multiclassing rules are explained in the Player's Handbook on page 163 :

Multiclassing lets you combine classes together [...]. Your DM
decides whether these options are available in a campaign.

So check with your DM if they are OK allowing this.

Multiclassing allows you to gain levels in multiple classes. Doing so
lets you mix the abilities of those classes to realize a character
concept that might not be reflected in one of the standard class
options. With this rule, you have the option of gaining a level in a
new class whenever you advance in level, instead of gaining a level in
your current class. Your levels in all your classes are added together
to determine your character level. For example, if you have three
levels in wizard and two in fighter, you're a 5th-level character.

So you can take 2 levels of Warlock, then 1 level of rogue (as long as you meet the 13 DEX prerequisite for multiclassing into Rogue and the 13 CHA needed to multiclass out of warlock)(thanks @Someone_Evil).

Doing so will give you the following proficiencies :

Light armor, one skill from the class's skill list, thieves' tools

As well as expertise in two skills, Thieves' Cant, and Sneak Attack (thanks @Davo)

To add to the answers already posted:
When you gain a Level, you may (assuming your GM is allowing multi-classing), choose any Class that you qualify for (by virtue of 13+ Ability Scores).
Multi-classing Rules here

So you may begin as a Warlock, take x Warlock levels, then a Rogue Level, and then more Warlock levels (and maybe a Sorcerer level, if you so choose, as you meet the 13 CHA*).

You can swap back and forth between any classes that you qualify for.
e.g. if you somehow got your STR to 13, you could then take a level of Barbarian, and then more Warlock, or Rogue levels!

*Not that I would recommend this, but it's your character :)