It depends on what you want to say.

1) If you are expressing something that happens in the present, you must use the present tense in both clauses.

For example: si j'ai faim, j'achète beaucoup de chocolat (if/when I'm hungry, I buy lots of chocolate)

2) If you are expressing something that will probably happen in the future, you must use the present tense in the si clause and the future tense in the next clause.

For example: si je vais en Espagne cet été, je pourrai profiter de la vie nocturne (If I go to Spain this summer, I'll be able to enjoy the nightlife)

3) If you are expressing something that may or may not happen, you must use the imperfect tense in the si clause and the conditional tense in the next clause.

For example: si j'avais l'argent, je ferais le tour du monde (If I had the money, I would travel the world)

4) If you are expressing something that may have happened but didn't, you must use the pluperfect tense in the si clause and the conditional perfect in the next clause.

For example: si j'étais allé à la fête hier soir, j'aurais bu beaucoup d'alcool (If I had been to the party last night, I would have drunk a lot of alcohol)

5) The future and the conditional tenses can also be used in a si clause in direct speech and when si takes the meaning of whether.

For example:

a) Hier ma mère m'a demandé si mon petit ami serait chez nous aujourd'hui (Yesterday my mum asked my if my boyfriend would be at our house today)

b) Je ne sais pas si je réussirai à mes examens (I don't know whether I'll pass my exams)