When it comes to dating knowing whether or not a person is available is one of the most important things on the checklist.

If you want to ask if someone is single you would use the word soltero for a male and soltera for a female:

Are you single?
¿Eres soltero/a?
¿EH-rehs sohl-TEH-roh/ah?

When asking if someone is single, you use a conjugation of ser or estar as the verb. Depending on the area where you are, you might find one used more than the other, but they are generally interchangeable in this context.

He/She is single.
Está soltero/a. (estar)
eh-STAH sohl-TEH-roh/ah.

He/She is single.
Es soltero/a. (ser)
ehs sohl-TEH-roh/ah.

Solterón (sohl-teh-ROHN) for males and solterona (sohl-teh-ROHN-ah) for females are a stronger way to say soltero/a but their meanings are quite different.

If a male is said to be a solterón this could mean that they are either a confirmed or long term bachelor. This isn't to say that they might never want to settle down, but chances are they are more of a player than someone interested in commitment.

On the other hand solterona tends to categorize a female being an "old maid" which is much more derogatory than the male version, so you wouldn't want to ask someone ¿Eres solterona? or you might be wearing a drink instead of sharing one.
 

Here's a mistake to avoid.
 
If you've learned some Spanish you probably know that the translation of the word "of" is de and the word "about" can be acerca de or sobre.
 
When you're dating someone it's popular to tell them that you've been thinking "about" or "of" them.
 
I've been thinking about you.
 
I always think about you.
 
I think about you all the time.
 
In Spanish when it comes to "thinking about you" or "thinking of you" the literal translation isn't used. Instead of translating the word "about" or "of" the word en is used. En means "in" or "on" but in these examples it's the correct translation.
 
So:
 
I've been thinking about you.
He estado pensando en ti.
eh ehs-tah-doh pehn-sahn-doh ehn tee.
 
I always think about you.
Siempre pienso en ti.
see-ehm-preh pee-ehn-soh ehn tee.
 
I think about you all the time.
Pienso en ti todo el tiempo.
pee-ehn-soh ehn tee toh-doh ehl tee-ehm-poh.

The literal translation of the above phrases is "Are you accompanied?"

Here's a couple ways to ask if someone is traveling alone or with others:

A couple responses to any of the above:

I’m by myself.
Estoy solo/a. (o= for males, a= for females)
ehs-toy soh-loh/ah.

There are two ways to say "I miss you" in Spanish that are the most common. Which one you will hear more depends whether you are in Spain or Latin America.

The most common way to say "I miss you" in Latin America uses the verb extrañar:

I miss you.
Te extraño.
teh ehks-trahn-yoh.

The most common way to say "I miss you" in Spain uses the verb echar:

I miss you.
Te echo de menos.
teh eh-choh deh meh-nohs.

Here are two more ways to say it that aren't as common as the above phrases:

I miss you. / I need you.
Me haces falta.
meh ah-sehs fahl-tah.

The literal translation of the above phrase doesn't translate very well to English. It uses the word falta which means "lack." It translates more to "I need you" but it is commonly used to say "I miss you" and means basically the same thing.

Here is another way to say "I miss you":

I miss you. / I long for you.
Te añoro.
teh ahn-yoh-roh.

The above phrase is not very common but says "I miss you" with a little more flair. It's like saying "I long for you."

I'll mention a couple mistakes to watch out for.  

Remember if you are talking about “missing” a bus or plane because you did not make it on time use the verb perder instead of the above verbs:

I missed the plane because I woke up late.
Perdí el avión porque yo me desperté tarde.
Pehr-DEE ehl ah-vee-OHN pohr-keh meh dehs-pehr-TEH tahr-deh.

If you want to say you “missed out” on some event like a party or soccer game you can use the reflexive version of perder which is perderse:

I missed out on the party.
Me perdí la fiesta.
Meh pehr-DEE lah fee-esh-tah.

I want to throw a link at this woman living in Panama and observing the male tourists hitting on the local chicas… in English. You've probably seen this if you've spent time in tourist spots in Latin America. I’ve witnessed this go down more than a few times. Accurate and good for a chuckle. Like she says though, at least they're giving it a shot.

When it comes to sexual phrases in Spanish a lot of countries have their own specific slang and terms. With that in mind I’ve tried to list phrases here that are common and understood in any Spanish speaking country. Here’s how you can tell someone how turned on you are:

You are really turning me on.
Me estás poniendo muy caliente.
meh ehs-tAHs poh-nee-ehn-doh mwee kah-lee-ehn-teh.

In Spanish it’s more common to say poniendo “putting” as in “putting me hot” or “putting me horny” than “making me hot,” “making me horny” or "turning me on."

Just like in English there’s a pretty wide range of ways to say you want to have sex:

From:

Make love to me.
Hazme el amor.
ahs-meh ehl ah-mohr.

To:

Let’s fuck.
Vamos a coger. (common in Latin America)
vah-mohs ah koh-hehr.

Let’s fuck.
Vamos a follar. (common in Spain)
vah-mohs ah foh-yahr.

Here’s a couple things you can say during sex when you want to say how good something feels:

Me encanta.
meh ehn-kahn-tah.

Qué rico.
kEH ree-koh.

Everyone likes compliments about how great they are in bed. So here’s a couple you can look forward to using. Keep in mind though if you’re going to be having sex with them again, you compliment them, and you actually didn’t like it that much you’ll probably wind up with the same thing next time.

With that said, here’s a couple ways to compliment someone’s performance:

You're great in bed.
Eres fabuloso/a en la cama.
eh-rehs fah-boo-loh-soh/ah ehn lah kah-mah.

And if your eyes got dislodged from their sockets from the intensity…

That was the best sex I've ever had.
Ha sido el mejor sexo de mi vida.
ah see-doh ehl meh-hohr sehks-oh deh mee vee-dah.