Personal loan rates are at all time lows. Ads shout about deals for as little as 2.9%. Yet debt is like fire. Done well it's a useful tool, done badly it burns. And sadly it’s a cautionary tale from a recent caller to my This Morning phone-in that prompted me to write this.

Last summer, Catherine thought she was doing right by using a credit reference agency's eligibility tool, to find a cheap loan for £1,000, "It came up saying, 'you can get accepted' - I thought, 'brilliant, I can fix my car'. [Now, having such a costly loan on my credit file has] affected my remortgage. The loan was 54.9% over 5yrs... to pay it off now would be £2,800."

These numbers don't add up, so one way or another she's having terrible loan issues at an extortionate cost. So here's my 6 key need-to-knows.

1. Establish if you really need to borrow?

The most important start point though for anyone isn’t how to borrow, it’s whether you should. If you’re already in debt and struggling, you can't borrow your way out of it - instead it’s best to get free debt counselling from charities citizensadvice.org.uk, nationaldebtline.org, or stepchange.org or if you need emotional support too, try capuk.org.

Yet sometimes borrowing is a valid decision – it may be you need car insurance and to pay it upfront is better than monthly (for that I’d carefully use a 0% credit card). Or there’s a big one off purchase for your home such as a new sofa, or even kitchen – and waiting for it may be a life detriment. If you do need to borrow, do always minimise the amount, budget to ensure repayments are affordable, and repay as quickly as possible.

2. Before applying use an eligibility checker

Using a loans eligibility calculator is the right starting point. You tell it your information and it shows you which loan you are most likely to be accepted for. It does that without impacting your credit worthiness – letting you home in on the right loan before applying. Some lenders have their own ones, or there are comparison sites to do it over a whole range of loans in one.

Yet the fact you can get a loan doesn’t mean you should. As Catherine found, some eligibility checkers include both personal loans and more unsavoury high interest loans (for the sake of transparency mine at www.moneysavingexpert.com/LoansCalc excludes high interest loans) otherwise you can end up in the sub-prime market without realising it.

Loan acceptance is about far more than just credit history - income plays a key role. So if you’ve a good credit score, while a lender may accept you if you want to borrow £2,000, if your income isn’t enough it may reject you for £5,000.

If you can’t get cheap lending, it often means most lenders don’t think you can afford it. So rather than jumping on to a higher interest loan – see it as a warning sign against borrowing.

3. Loan rates are at all-time lows

If you’ve a decent credit score, loan rates are at near all-time lows. Here are the quick best buys (though better to go through an eligibility calc to see what you’ll be accepted for).

£1,000 - £1,999: admiral.com is 13.2% rep APR

£2,000 - £2,999: zopa.com is 12.9% rep APR and admiral.com is 13.2% rep APR

£3,000 - £4,999: admiral.com is 6.4% rep APR

£5,000 - £7,499: zopa.com and ratesetter.com are 3.3% rep APR and admiral.com is 3.4% rep APR

£7,500 - £15,000 bank.marksandspencer.com is 2.9% rep APR

4. Not everyone will get the advertised rate

As you’ll see from the best-buys, all loans are ‘representative APRs’. The ‘representative’ bit sadly means only 51% of people who apply will get the advertised rate. The rest can be charged more, and there's no limit how much more.

So a loan advertised at 2.9% could end up being 20%. You're almost always only told this after application, so after you’ve been accepted ALWAYS CHECK THE RATE YOU’RE GIVEN, even if you’re using an eligibility checker. Again, I suspect this happened to Catherine.

5. The longer you borrow, the more you repay

As with many things, with loans, length matters. The longer you take to repay, the more interest accrues. Eg, a 15% one-year loan costs far less interest than a 4% loan over 5 years. So only borrow for the length that you absolutely need to repay in. If you can reduce the term even by a year that’ll make a big difference to the interest you pay.

6. If borrowing under £3,000, you might be able to get a 0% loan

If what you need to borrow for can be paid on a 0% credit card, then get one of those, and as long as you clear it before the 0% ends and don’t miss repayments, there’s no cost. Yet the key is will your credit limit be big enough – getting over £3,000 is tough for many.

Even if you can’t pay on the card, there’s a way to get a 0% credit card loan. You do this via a few specialist cards that offer ‘money transfers’. These cards let you transfer cash from the credit card to your bank account for a small nominal fee, so then you owe the card at 0%. These are likely to be the cheapest route for smaller loans.

For example MBNA is currently giving up to 28 months 0% for a 2.99% fee – so if you want to borrow £2,000 that’s a £60 fee loan - just make sure you clear the card before the 0% ends, or you pay 23.93% after. If you are doing this it is worth doing a little bit of reading though as money transfers can be complex – see my guide at www.moneysavingexpert.com/moneytransfers

Martin Lewis is the Founder and Chair of MoneySavingExpert.com. To join the 13 million people who get his free Money Tips weekly email, go to www.moneysavingexpert.com/latesttip