When we make the conditional in English we just use the word if and the verb in past form ex: if I was... well the structure in spanish language is similar, we use the word "si" instead of word "if" and instead of verb in past form we use the verb in pretérito imperfecto de subjuntivo. What is that?? well is a verbal time that does not exist in english so you just have to learn, here there is a good site with all spanish verbal times , have a look at pretérito imperfecto de modo subjuntivo and lets start to build conditionals:
At first check out on this site the pretérito imperfecto de modo subjuntivo: http://www.verbolog.com/conjuga.htm and also read the lesson 16
If I had money......si tuviera dinero......
If I was a child.....si fuera niño.......
If you were here...si estuvueras aquí.....
I'm sorry but the only way is learn the verbal time but try with the most important tener, hacer,poder...
Now, and after had read the lesson 16 here lesson 16 you are able to build a full conditional tense:
If I had money I would buy a car.....si tuviera dinero compraría un coche
If I was a child I would be playing....si fuera niño estaría jugando
If you were here I would be happy...si estuvieras aquí estaría feliz
Structure: "si + verb in pretérito imperfecto de subjuntivo + (verb conditioned(as we saw in lesson 16))
here there are some examples of pretérito imperfecto de subjuntivo here
Comer=to eat comiera=from comer but pretérito imperfecto de subjuntivo. Estaría=from verb estar but conditioned as we saw in lesson 16.
"si yo comiera no estaría tan delgado" ..."If I ate i would not be that thin"
REALLY IMPORTANT: do not translate just keep the idea, the pretérito imperfecto de subjuntivo does not exist in english!!!