Tax Credits are divided into :

Working Tax Credits
and Child Tax Credits

The first port of call would be the HMRC website:

http://www.taxcredits.inlandrevenue.gov.uk/HomeNew.aspx

Here you can answer a short survey to see if you are entitled:

http://www.taxcredits.inlandrevenue....Household.aspx

BUT, take note of Immigration restrictions:

http://www.taxcredits.inlandrevenue....ov.uk&v2=&tmt=

Subject to Immigration Control
You are subject to immigration control if

• the Home Office says your leave to remain in the United Kingdom (UK) is on the condition that you do not have recourse to public funds, or

• you require leave to enter or remain in the UK, but do not have it.

If you are subject to immigration control, you may still be entitled to tax credits if

• you are claiming as a couple and only one of you is subject to immigration control

• you are a family member of a country in the European Economic Area (EEA) or of Switzerland
• you are a sponsored immigrant under the Home Office rules
• you (or your partner) are legally working in the UK and are nationals of Algeria, Morocco, San Marino, Tunisia or Turkey.

You are
not subject to immigration control if you are

• a UK national
• a national of another country in the EEA or of Switzerland
someone who has been granted indefinite leave to remain in the UK (including those granted humanitarian protection or discretionary leave, previously exceptional leave)
• someone who has been granted asylum in the UK.

You can claim online, but as usual, the online system is kaput, so claim forms can be obtained from Job Centre Plus or by phone on 08453003909.

As you can see on the underlined sentences, if your Wife/husband or Fiance/Fiancee does not have ILR, only the husband/wife Fiance/Fiancee can apply.